From 2ebce5610a61e109217ad6629b08e52b5a3105c4 Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Tue, 28 Oct 2025 09:17:28 -0300
Subject: [PATCH 1/8] chore(message-reader): add :feature:mail:message:reader
 module

---
 feature/mail/message/reader/api/build.gradle.kts  | 14 ++++++++++++++
 .../feature/mail/message/reader/api/.gitkeep      |  0
 feature/mail/message/reader/impl/build.gradle.kts | 15 +++++++++++++++
 .../feature/mail/message/reader/impl/.gitkeep     |  0
 settings.gradle.kts                               |  2 ++
 5 files changed, 31 insertions(+)
 create mode 100644 feature/mail/message/reader/api/build.gradle.kts
 create mode 100644 feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/.gitkeep
 create mode 100644 feature/mail/message/reader/impl/build.gradle.kts
 create mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/.gitkeep

diff --git a/feature/mail/message/reader/api/build.gradle.kts b/feature/mail/message/reader/api/build.gradle.kts
new file mode 100644
index 00000000000..36dd498f42b
--- /dev/null
+++ b/feature/mail/message/reader/api/build.gradle.kts
@@ -0,0 +1,14 @@
+plugins {
+    id(ThunderbirdPlugins.Library.kmp)
+}
+
+kotlin {
+    sourceSets {
+        commonMain.dependencies {
+        }
+    }
+}
+
+android {
+    namespace = "net.thunderbird.feature.mail.message.reader.api"
+}
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/.gitkeep b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/.gitkeep
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/feature/mail/message/reader/impl/build.gradle.kts b/feature/mail/message/reader/impl/build.gradle.kts
new file mode 100644
index 00000000000..6d94c5abee4
--- /dev/null
+++ b/feature/mail/message/reader/impl/build.gradle.kts
@@ -0,0 +1,15 @@
+plugins {
+    id(ThunderbirdPlugins.Library.kmp)
+}
+
+kotlin {
+    sourceSets {
+        commonMain.dependencies {
+            implementation(projects.feature.mail.message.reader.api)
+        }
+    }
+}
+
+android {
+    namespace = "net.thunderbird.feature.mail.message.reader.impl"
+}
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/.gitkeep b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/.gitkeep
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/settings.gradle.kts b/settings.gradle.kts
index e39595534d8..16f8cb1c8da 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -101,6 +101,8 @@ include(
     ":feature:mail:message:list",
     ":feature:mail:message:export:api",
     ":feature:mail:message:export:impl-eml",
+    ":feature:mail:message:reader:api",
+    ":feature:mail:message:reader:impl",
 )
 
 include(

From 77af91ce7dd2a51bdbe0c932c5c0c66aab05d75b Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Tue, 28 Oct 2025 09:37:25 -0300
Subject: [PATCH 2/8] chore(message-reader): enable webview debugging on
 development builds

---
 .../ui/legacy/src/main/java/com/fsck/k9/view/MessageWebView.kt  | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/view/MessageWebView.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/view/MessageWebView.kt
index 6555c4a966e..ff121d80162 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/view/MessageWebView.kt
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/view/MessageWebView.kt
@@ -6,6 +6,7 @@ import android.util.AttributeSet
 import android.webkit.WebSettings.LayoutAlgorithm
 import android.webkit.WebSettings.RenderPriority
 import android.webkit.WebView
+import com.fsck.k9.core.BuildConfig
 import com.fsck.k9.mailstore.AttachmentResolver
 import net.thunderbird.core.android.common.view.showInDarkMode
 import net.thunderbird.core.android.common.view.showInLightMode
@@ -62,6 +63,7 @@ class MessageWebView : WebView, KoinComponent {
 
         // Disable network images by default. This is overridden by preferences.
         blockNetworkData(true)
+        setWebContentsDebuggingEnabled(BuildConfig.DEBUG)
     }
 
     private fun configureDarkLightMode(

From e32bc2e3e266316491cd4f15985588dd13b08a92 Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Thu, 6 Nov 2025 12:54:11 -0400
Subject: [PATCH 3/8] chore(message-reader): add
 use_new_message_reader_css_styles feature flag

---
 app-k9mail/build.gradle.kts                                | 1 +
 .../kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt  | 2 ++
 .../kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt  | 2 ++
 app-thunderbird/build.gradle.kts                           | 1 +
 .../android/featureflag/TbFeatureFlagFactory.kt            | 2 ++
 .../android/featureflag/TbFeatureFlagFactory.kt            | 2 ++
 .../android/featureflag/TbFeatureFlagFactory.kt            | 2 ++
 .../android/featureflag/TbFeatureFlagFactory.kt            | 2 ++
 feature/mail/message/reader/api/build.gradle.kts           | 2 ++
 .../thunderbird/feature/mail/message/reader/api/.gitkeep   | 0
 .../mail/message/reader/api/MessageReaderFeatureFlags.kt   | 7 +++++++
 11 files changed, 23 insertions(+)
 delete mode 100644 feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/.gitkeep
 create mode 100644 feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt

diff --git a/app-k9mail/build.gradle.kts b/app-k9mail/build.gradle.kts
index ecad942e649..1e1ae615999 100644
--- a/app-k9mail/build.gradle.kts
+++ b/app-k9mail/build.gradle.kts
@@ -142,6 +142,7 @@ dependencies {
     implementation(projects.core.ui.legacy.theme2.k9mail)
     implementation(projects.feature.launcher)
     implementation(projects.feature.mail.message.list)
+    implementation(projects.feature.mail.message.reader.api)
 
     implementation(projects.legacy.core)
     implementation(projects.legacy.ui.legacy)
diff --git a/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt b/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
index 71b0651713f..64b58c3eda6 100644
--- a/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
+++ b/app-k9mail/src/debug/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
@@ -7,6 +7,7 @@ import net.thunderbird.core.featureflag.FeatureFlagFactory
 import net.thunderbird.core.featureflag.FeatureFlagKey
 import net.thunderbird.core.featureflag.toFeatureFlagKey
 import net.thunderbird.feature.account.settings.AccountSettingsFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 
 class K9FeatureFlagFactory : FeatureFlagFactory {
     override fun createFeatureCatalog(): List<FeatureFlag> {
@@ -20,6 +21,7 @@ class K9FeatureFlagFactory : FeatureFlagFactory {
             FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
             FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = true),
             FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = false),
+            FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = true),
         )
     }
 }
diff --git a/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt b/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
index 9a39c1364ba..194e533466e 100644
--- a/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
+++ b/app-k9mail/src/release/kotlin/app/k9mail/featureflag/K9FeatureFlagFactory.kt
@@ -7,6 +7,7 @@ import net.thunderbird.core.featureflag.FeatureFlagFactory
 import net.thunderbird.core.featureflag.FeatureFlagKey
 import net.thunderbird.core.featureflag.toFeatureFlagKey
 import net.thunderbird.feature.account.settings.AccountSettingsFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 
 /**
  * Feature flags for K-9 Mail (release)
@@ -23,6 +24,7 @@ class K9FeatureFlagFactory : FeatureFlagFactory {
             FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
             FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = false),
             FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = false),
+            FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = false),
         )
     }
 }
diff --git a/app-thunderbird/build.gradle.kts b/app-thunderbird/build.gradle.kts
index 6e36feb3d75..9e7b514092e 100644
--- a/app-thunderbird/build.gradle.kts
+++ b/app-thunderbird/build.gradle.kts
@@ -222,6 +222,7 @@ dependencies {
 
     implementation(projects.feature.account.settings.impl)
     implementation(projects.feature.mail.message.list)
+    implementation(projects.feature.mail.message.reader.api)
 
     implementation(projects.feature.widget.messageList)
     implementation(projects.feature.widget.messageListGlance)
diff --git a/app-thunderbird/src/beta/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt b/app-thunderbird/src/beta/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
index 3b5526ed6ee..1d2ad5f28b0 100644
--- a/app-thunderbird/src/beta/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
+++ b/app-thunderbird/src/beta/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
@@ -7,6 +7,7 @@ import net.thunderbird.core.featureflag.FeatureFlagFactory
 import net.thunderbird.core.featureflag.FeatureFlagKey
 import net.thunderbird.core.featureflag.toFeatureFlagKey
 import net.thunderbird.feature.account.settings.AccountSettingsFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 
 /**
  * Feature flags for Thunderbird Beta
@@ -23,6 +24,7 @@ class TbFeatureFlagFactory : FeatureFlagFactory {
             FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
             FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = false),
             FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = false),
+            FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = false),
         )
     }
 }
diff --git a/app-thunderbird/src/daily/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt b/app-thunderbird/src/daily/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
index f0f957042e3..1b09198e0ef 100644
--- a/app-thunderbird/src/daily/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
+++ b/app-thunderbird/src/daily/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
@@ -7,6 +7,7 @@ import net.thunderbird.core.featureflag.FeatureFlagFactory
 import net.thunderbird.core.featureflag.FeatureFlagKey
 import net.thunderbird.core.featureflag.toFeatureFlagKey
 import net.thunderbird.feature.account.settings.AccountSettingsFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 
 /**
  * Feature flags for Thunderbird Daily
@@ -23,6 +24,7 @@ class TbFeatureFlagFactory : FeatureFlagFactory {
             FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
             FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = true),
             FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = false),
+            FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = true),
         )
     }
 }
diff --git a/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt b/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
index 498e87865db..0daf653fe54 100644
--- a/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
+++ b/app-thunderbird/src/debug/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
@@ -7,6 +7,7 @@ import net.thunderbird.core.featureflag.FeatureFlagFactory
 import net.thunderbird.core.featureflag.FeatureFlagKey
 import net.thunderbird.core.featureflag.toFeatureFlagKey
 import net.thunderbird.feature.account.settings.AccountSettingsFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 
 /**
  * Feature flags for Thunderbird Debug
@@ -23,6 +24,7 @@ class TbFeatureFlagFactory : FeatureFlagFactory {
             FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
             FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = true),
             FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = false),
+            FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = true),
         )
     }
 }
diff --git a/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt b/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
index 470e84e59c0..c6f0c59c2fb 100644
--- a/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
+++ b/app-thunderbird/src/release/kotlin/net/thunderbird/android/featureflag/TbFeatureFlagFactory.kt
@@ -7,6 +7,7 @@ import net.thunderbird.core.featureflag.FeatureFlagFactory
 import net.thunderbird.core.featureflag.FeatureFlagKey
 import net.thunderbird.core.featureflag.toFeatureFlagKey
 import net.thunderbird.feature.account.settings.AccountSettingsFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 
 /**
  * Feature flags for Thunderbird (release)
@@ -23,6 +24,7 @@ class TbFeatureFlagFactory : FeatureFlagFactory {
             FeatureFlag(MessageListFeatureFlags.UseComposeForMessageListItems, enabled = false),
             FeatureFlag(MessageViewFeatureFlags.ActionExportEml, enabled = false),
             FeatureFlag(AccountSettingsFeatureFlags.EnableAvatarCustomization, enabled = false),
+            FeatureFlag(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles, enabled = false),
         )
     }
 }
diff --git a/feature/mail/message/reader/api/build.gradle.kts b/feature/mail/message/reader/api/build.gradle.kts
index 36dd498f42b..76d4b7b5e65 100644
--- a/feature/mail/message/reader/api/build.gradle.kts
+++ b/feature/mail/message/reader/api/build.gradle.kts
@@ -5,6 +5,8 @@ plugins {
 kotlin {
     sourceSets {
         commonMain.dependencies {
+            implementation(projects.core.common)
+            implementation(projects.core.featureflag)
         }
     }
 }
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/.gitkeep b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/.gitkeep
deleted file mode 100644
index e69de29bb2d..00000000000
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt
new file mode 100644
index 00000000000..e093461c43d
--- /dev/null
+++ b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/MessageReaderFeatureFlags.kt
@@ -0,0 +1,7 @@
+package net.thunderbird.feature.mail.message.reader.api
+
+import net.thunderbird.core.featureflag.FeatureFlagKey
+
+object MessageReaderFeatureFlags {
+    val UseNewMessageReaderCssStyles = FeatureFlagKey("use_new_message_reader_css_styles")
+}

From 0fefd2d9074f0c15ac1a31cef51e3fa0330fed14 Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Thu, 6 Nov 2025 12:57:58 -0400
Subject: [PATCH 4/8] feat(message-reader): add css classname provider

---
 app-common/build.gradle.kts                   |  2 +
 .../api/css/DefaultCssClassNameProvider.kt    | 26 +++++++++++++
 .../app/k9mail/feature/FeatureModule.kt       | 10 +++++
 .../android/feature/FeatureModule.kt          | 10 +++++
 .../reader/api/css/CssClassNameProvider.kt    | 37 +++++++++++++++++++
 5 files changed, 85 insertions(+)
 create mode 100644 app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
 create mode 100644 feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssClassNameProvider.kt

diff --git a/app-common/build.gradle.kts b/app-common/build.gradle.kts
index 373e8e396a0..4e6e3f97476 100644
--- a/app-common/build.gradle.kts
+++ b/app-common/build.gradle.kts
@@ -49,6 +49,8 @@ dependencies {
 
     implementation(projects.feature.mail.message.export.api)
     implementation(projects.feature.mail.message.export.implEml)
+    implementation(projects.feature.mail.message.reader.api)
+    implementation(projects.feature.mail.message.reader.impl)
 
     implementation(projects.mail.protocols.imap)
     implementation(projects.backend.imap)
diff --git a/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt b/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
new file mode 100644
index 00000000000..59d9bdf013b
--- /dev/null
+++ b/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
@@ -0,0 +1,26 @@
+package net.thunderbird.android.feature.mail.message.reader.api.css
+
+import com.fsck.k9.message.html.EmailTextToHtml
+import net.thunderbird.core.featureflag.FeatureFlagProvider
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+
+class DefaultCssClassNameProvider(
+    featureFlagProvider: FeatureFlagProvider,
+    override val defaultNamespaceClassName: String,
+) : CssClassNameProvider {
+    override val rootClassName: String = "${defaultNamespaceClassName}__message-viewer"
+    override val mainContentClassName: String = "${defaultNamespaceClassName}__main-content"
+    override val plainTextMessagePreClassName: String =
+        if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
+            EmailTextToHtml.K9MAIL_CSS_CLASS
+        } else {
+            "${defaultNamespaceClassName}__plain-text-message-pre"
+        }
+    override val signatureClassName: String =
+        if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
+            "${defaultNamespaceClassName}__signature"
+        } else {
+            "k9mail-signature"
+        }
+}
diff --git a/app-k9mail/src/main/kotlin/app/k9mail/feature/FeatureModule.kt b/app-k9mail/src/main/kotlin/app/k9mail/feature/FeatureModule.kt
index 5fd16ed15e1..93ddebcabd8 100644
--- a/app-k9mail/src/main/kotlin/app/k9mail/feature/FeatureModule.kt
+++ b/app-k9mail/src/main/kotlin/app/k9mail/feature/FeatureModule.kt
@@ -5,8 +5,11 @@ import app.k9mail.feature.funding.featureFundingModule
 import app.k9mail.feature.migration.launcher.featureMigrationModule
 import app.k9mail.feature.onboarding.migration.onboardingMigrationModule
 import app.k9mail.feature.telemetry.telemetryModule
+import com.fsck.k9.BuildConfig
+import net.thunderbird.android.feature.mail.message.reader.api.css.DefaultCssClassNameProvider
 import net.thunderbird.feature.account.settings.featureAccountSettingsModule
 import net.thunderbird.feature.mail.message.list.featureMessageListModule
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import org.koin.dsl.module
 
 val featureModule = module {
@@ -18,4 +21,11 @@ val featureModule = module {
     includes(featureMessageListModule)
 
     single<FundingSettings> { K9FundingSettings() }
+
+    single<CssClassNameProvider> {
+        DefaultCssClassNameProvider(
+            featureFlagProvider = get(),
+            defaultNamespaceClassName = BuildConfig.APPLICATION_ID.replace(".", "-"),
+        )
+    }
 }
diff --git a/app-thunderbird/src/main/kotlin/net/thunderbird/android/feature/FeatureModule.kt b/app-thunderbird/src/main/kotlin/net/thunderbird/android/feature/FeatureModule.kt
index 82a9250486c..bd5df1fe0b4 100644
--- a/app-thunderbird/src/main/kotlin/net/thunderbird/android/feature/FeatureModule.kt
+++ b/app-thunderbird/src/main/kotlin/net/thunderbird/android/feature/FeatureModule.kt
@@ -5,8 +5,11 @@ import app.k9mail.feature.funding.featureFundingModule
 import app.k9mail.feature.migration.launcher.featureMigrationModule
 import app.k9mail.feature.onboarding.migration.onboardingMigrationModule
 import app.k9mail.feature.telemetry.telemetryModule
+import net.thunderbird.android.BuildConfig
+import net.thunderbird.android.feature.mail.message.reader.api.css.DefaultCssClassNameProvider
 import net.thunderbird.feature.account.settings.featureAccountSettingsModule
 import net.thunderbird.feature.mail.message.list.featureMessageListModule
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import org.koin.dsl.module
 
 internal val featureModule = module {
@@ -18,4 +21,11 @@ internal val featureModule = module {
     includes(featureMessageListModule)
 
     single<FundingSettings> { TbFundingSettings() }
+
+    single<CssClassNameProvider> {
+        DefaultCssClassNameProvider(
+            featureFlagProvider = get(),
+            defaultNamespaceClassName = BuildConfig.APPLICATION_ID.replace(".", "-"),
+        )
+    }
 }
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssClassNameProvider.kt b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssClassNameProvider.kt
new file mode 100644
index 00000000000..76be3c1d750
--- /dev/null
+++ b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssClassNameProvider.kt
@@ -0,0 +1,37 @@
+package net.thunderbird.feature.mail.message.reader.api.css
+
+/**
+ * Provides CSS class names used for styling the message viewer.
+ *
+ * This allows for a consistent and centralized way to manage the class names
+ * used in the HTML of the message display, making it easier to style and
+ * maintain.
+ */
+interface CssClassNameProvider {
+    /**
+     * The class name used to namespace all CSS rules to avoid conflicts with message content.
+     *
+     * This class should be applied to a high-level container element wrapping the entire message view.
+     */
+    val defaultNamespaceClassName: String
+
+    /**
+     * The class name for the root element of the message content. This is typically the `<body>` tag.
+     */
+    val rootClassName: String
+
+    /**
+     * The class name for the main content block of the message viewer.
+     */
+    val mainContentClassName: String
+
+    /**
+     * The class name for the `<pre>` tag that wraps plain text messages to preserve formatting.
+     */
+    val plainTextMessagePreClassName: String
+
+    /**
+     * The class name used to style email signatures.
+     */
+    val signatureClassName: String
+}

From 43a85eb7f4bc34856f401f27277203492eaf5339 Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Fri, 7 Nov 2025 10:45:08 -0400
Subject: [PATCH 5/8] feat(message-reader): add css variable name provider

---
 .../reader/api/css/CssVariableNameProvider.kt      | 14 ++++++++++++++
 .../feature/mail/message/reader/impl/.gitkeep      |  0
 .../impl/css/DefaultCssVariableNameProvider.kt     | 11 +++++++++++
 3 files changed, 25 insertions(+)
 create mode 100644 feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssVariableNameProvider.kt
 delete mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/.gitkeep
 create mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultCssVariableNameProvider.kt

diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssVariableNameProvider.kt b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssVariableNameProvider.kt
new file mode 100644
index 00000000000..8fb9acef9fc
--- /dev/null
+++ b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssVariableNameProvider.kt
@@ -0,0 +1,14 @@
+package net.thunderbird.feature.mail.message.reader.api.css
+
+/**
+ * Provides CSS variable values for theming the message viewer.
+ *
+ * Implementations of this interface supply the name of the variables (e.g., "--my-variable")
+ * that will be used in the message display's stylesheet.
+ */
+interface CssVariableNameProvider {
+    /**
+     * The name of the CSS variable used to specify the 'border-left-color' for blockquote elements.
+     */
+    val blockquoteDefaultBorderLeftColor: String
+}
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/.gitkeep b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/.gitkeep
deleted file mode 100644
index e69de29bb2d..00000000000
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultCssVariableNameProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultCssVariableNameProvider.kt
new file mode 100644
index 00000000000..418e87399ac
--- /dev/null
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultCssVariableNameProvider.kt
@@ -0,0 +1,11 @@
+package net.thunderbird.feature.mail.message.reader.impl.css
+
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
+
+internal class DefaultCssVariableNameProvider(
+    cssClassNameProvider: CssClassNameProvider,
+) : CssVariableNameProvider {
+    override val blockquoteDefaultBorderLeftColor: String =
+        "--${cssClassNameProvider.defaultNamespaceClassName}__blockquote-default-border-color"
+}

From 1943c5e645da2d9017f13c577cbdddf9f9003a6f Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Fri, 7 Nov 2025 10:50:36 -0400
Subject: [PATCH 6/8] feat(message-reader): add css style providers

---
 .../common/feature/AppCommonFeatureModule.kt  |  2 +
 .../reader/api/css/CssStyleProvider.kt        | 50 +++++++++++++++++
 .../mail/message/reader/impl/build.gradle.kts |  4 ++
 .../impl/css/DefaultGlobalCssStyleProvider.kt | 56 +++++++++++++++++++
 ...inTextMessagePreElementCssStyleProvider.kt | 34 +++++++++++
 .../css/DefaultSignatureCssStyleProvider.kt   | 18 ++++++
 .../impl/inject/FeatureMessageReaderModule.kt | 33 +++++++++++
 7 files changed, 197 insertions(+)
 create mode 100644 feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt
 create mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
 create mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt
 create mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt
 create mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt

diff --git a/app-common/src/main/kotlin/net/thunderbird/app/common/feature/AppCommonFeatureModule.kt b/app-common/src/main/kotlin/net/thunderbird/app/common/feature/AppCommonFeatureModule.kt
index 3d8411b57ff..5220223895c 100644
--- a/app-common/src/main/kotlin/net/thunderbird/app/common/feature/AppCommonFeatureModule.kt
+++ b/app-common/src/main/kotlin/net/thunderbird/app/common/feature/AppCommonFeatureModule.kt
@@ -4,6 +4,7 @@ import app.k9mail.feature.launcher.FeatureLauncherExternalContract
 import app.k9mail.feature.launcher.di.featureLauncherModule
 import net.thunderbird.app.common.feature.mail.appCommonFeatureMailModule
 import net.thunderbird.feature.mail.message.composer.inject.featureMessageComposerModule
+import net.thunderbird.feature.mail.message.reader.impl.inject.featureMessageReaderModule
 import net.thunderbird.feature.navigation.drawer.api.NavigationDrawerExternalContract
 import net.thunderbird.feature.notification.impl.inject.featureNotificationModule
 import org.koin.android.ext.koin.androidContext
@@ -14,6 +15,7 @@ internal val appCommonFeatureModule = module {
     includes(featureNotificationModule)
     includes(featureMessageComposerModule)
     includes(appCommonFeatureMailModule)
+    includes(featureMessageReaderModule)
 
     factory<FeatureLauncherExternalContract.AccountSetupFinishedLauncher> {
         AccountSetupFinishedLauncher(
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt
new file mode 100644
index 00000000000..8ec618aa1cf
--- /dev/null
+++ b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt
@@ -0,0 +1,50 @@
+package net.thunderbird.feature.mail.message.reader.api.css
+
+/**
+ * Provides CSS styles to be injected into the message viewer's web view.
+ *
+ * This sealed interface serves as a common type for different CSS style providers.
+ * Each implementation is responsible for a specific part of the message styling,
+ * such as global styles, signature styles, or plain text formatting.
+ */
+sealed interface CssStyleProvider {
+    /**
+     * The CSS style tag with its content as a string.
+     *
+     * This string should contain valid CSS rules that can be injected into an HTML document (e.g., a message view).
+     *
+     * Example:
+     * ```
+     * "body { color: red; } a { text-decoration: none; }"
+     * ```
+     */
+    val style: String
+}
+
+/**
+ * Provides CSS styles that are applied to the entire message view.
+ *
+ * This is used for global styles that affect the overall appearance of the email content,
+ * such as font size, colors, and layout adjustments for the entire message body.
+ */
+interface GlobalCssStyleProvider : CssStyleProvider
+
+/**
+ * Provides CSS styles specifically for the `<pre>` element used to wrap plain text messages.
+ *
+ * This allows for custom styling of plain text content, such as setting the font family,
+ * size, and controlling word wrapping behavior to ensure readability.
+ */
+interface PlainTextMessagePreElementCssStyleProvider : CssStyleProvider {
+    interface Factory {
+        fun create(useFixedWidthFont: Boolean): PlainTextMessagePreElementCssStyleProvider
+    }
+}
+
+/**
+ * Provides CSS styles specifically for email signatures.
+ *
+ * This is used to apply custom styling to the signature block within an email message,
+ * allowing it to be visually distinct from the main message body.
+ */
+interface SignatureCssStyleProvider : CssStyleProvider
diff --git a/feature/mail/message/reader/impl/build.gradle.kts b/feature/mail/message/reader/impl/build.gradle.kts
index 6d94c5abee4..72874141d3e 100644
--- a/feature/mail/message/reader/impl/build.gradle.kts
+++ b/feature/mail/message/reader/impl/build.gradle.kts
@@ -5,6 +5,10 @@ plugins {
 kotlin {
     sourceSets {
         commonMain.dependencies {
+            implementation(projects.core.common)
+            implementation(projects.core.featureflag)
+            implementation(projects.core.preference.api)
+            implementation(projects.core.ui.theme.api)
             implementation(projects.feature.mail.message.reader.api)
         }
     }
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
new file mode 100644
index 00000000000..1768e910d24
--- /dev/null
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
@@ -0,0 +1,56 @@
+package net.thunderbird.feature.mail.message.reader.impl.css
+
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.GlobalCssStyleProvider
+import org.intellij.lang.annotations.Language
+
+internal class DefaultGlobalCssStyleProvider(
+    cssClassNameProvider: CssClassNameProvider,
+    cssVariableNameProvider: CssVariableNameProvider,
+) : GlobalCssStyleProvider {
+    @Language("HTML")
+    override val style: String = """
+        |<style>
+        |  body { font-size: 0.9rem; }
+        |  .clear:after {
+        |    content: "";
+        |    clear: both;
+        |    display: block;
+        |  }
+        |  .${cssClassNameProvider.rootClassName} {
+        |    display: block;
+        |    user-select: auto;
+        |    -webkit-user-select: auto;
+        |  }
+        |  .${cssClassNameProvider.rootClassName}.${cssClassNameProvider.mainContentClassName} {
+        |    width: 100%;
+        |    overflow-wrap: break-word;
+        |  }
+        |  .${cssClassNameProvider.rootClassName}.${cssClassNameProvider.mainContentClassName} pre {
+        |    white-space: pre-wrap;
+        |  }
+        |  .${cssClassNameProvider.rootClassName}.${cssClassNameProvider.mainContentClassName} blockquote {
+        |    margin: auto 0 auto 0.8ex !important;
+        |    padding-left: 1ex !important;
+        |    border-left-width: 1px !important;
+        |    border-left-style: solid !important;
+        |    border-left-color: var(${cssVariableNameProvider.blockquoteDefaultBorderLeftColor}, #ccc);
+        |  }
+        |</style>
+    """.trimMargin()
+}
+
+internal class LegacyGlobalCssStyleProvider : GlobalCssStyleProvider {
+    @Language("HTML")
+    override val style: String = """
+        |<style type="text/css">
+        |  * {
+        |    background: #121212 !important;
+        |    color: #F3F3F3 !important
+        |  }
+        |  :link, :link * { color: #CCFF33 !important }
+        |  :visited, :visited * { color: #551A8B !important }
+        |</style>
+    """.trimMargin()
+}
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt
new file mode 100644
index 00000000000..2f15a167016
--- /dev/null
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt
@@ -0,0 +1,34 @@
+package net.thunderbird.feature.mail.message.reader.impl.css
+
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.PlainTextMessagePreElementCssStyleProvider
+import org.intellij.lang.annotations.Language
+
+class DefaultPlainTextMessagePreElementCssStyleProvider(
+    cssClassNameProvider: CssClassNameProvider,
+    useFixedWidthFont: Boolean,
+) : PlainTextMessagePreElementCssStyleProvider {
+
+    @Language("HTML")
+    override val style: String = """
+        |<style>
+        |  pre.${cssClassNameProvider.plainTextMessagePreClassName} {
+        |    white-space: pre-wrap;
+        |    word-wrap: break-word;
+        |    font-family: ${if (useFixedWidthFont) "monospace" else "sans-serif"};
+        |    margin-top: 0px;
+        |  }
+        |</style>
+    """.trimMargin()
+
+    class Factory(
+        private val cssClassNameProvider: CssClassNameProvider,
+    ) : PlainTextMessagePreElementCssStyleProvider.Factory {
+        override fun create(useFixedWidthFont: Boolean): PlainTextMessagePreElementCssStyleProvider {
+            return DefaultPlainTextMessagePreElementCssStyleProvider(
+                cssClassNameProvider = cssClassNameProvider,
+                useFixedWidthFont = useFixedWidthFont,
+            )
+        }
+    }
+}
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt
new file mode 100644
index 00000000000..1b959894da3
--- /dev/null
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt
@@ -0,0 +1,18 @@
+package net.thunderbird.feature.mail.message.reader.impl.css
+
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.SignatureCssStyleProvider
+import org.intellij.lang.annotations.Language
+
+class DefaultSignatureCssStyleProvider(
+    cssClassNameProvider: CssClassNameProvider,
+) : SignatureCssStyleProvider {
+    @Language("HTML")
+    override val style: String = """
+        |<style>
+        |  .${cssClassNameProvider.signatureClassName} {
+        |    opacity: 0.5;
+        |  }
+        |</style>
+    """.trimMargin()
+}
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
new file mode 100644
index 00000000000..7b9d5dbf7d3
--- /dev/null
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
@@ -0,0 +1,33 @@
+package net.thunderbird.feature.mail.message.reader.impl.inject
+
+import net.thunderbird.core.featureflag.FeatureFlagProvider
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.GlobalCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.css.PlainTextMessagePreElementCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.css.SignatureCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.impl.css.DefaultCssVariableNameProvider
+import net.thunderbird.feature.mail.message.reader.impl.css.DefaultGlobalCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.impl.css.DefaultPlainTextMessagePreElementCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.impl.css.DefaultSignatureCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.impl.css.LegacyGlobalCssStyleProvider
+import org.koin.dsl.module
+
+val featureMessageReaderModule = module {
+    single<CssVariableNameProvider> { DefaultCssVariableNameProvider(get()) }
+    single<GlobalCssStyleProvider> {
+        val featureFlagProvider = get<FeatureFlagProvider>()
+        if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
+            DefaultGlobalCssStyleProvider(
+                cssClassNameProvider = get(),
+                cssVariableNameProvider = get(),
+            )
+        } else {
+            LegacyGlobalCssStyleProvider()
+        }
+    }
+    single<SignatureCssStyleProvider> { DefaultSignatureCssStyleProvider(get()) }
+    factory<PlainTextMessagePreElementCssStyleProvider.Factory> {
+        DefaultPlainTextMessagePreElementCssStyleProvider.Factory(cssClassNameProvider = get())
+    }
+}

From dfdbeed8b495fe934805104f8920e1d1f055d5fa Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Fri, 7 Nov 2025 11:07:59 -0400
Subject: [PATCH 7/8] refactor(message-reader): use CSS providers instead of
 building css directly on DisplayHtml

The key changes include:
- Moving `HtmlSettings.kt` and `HtmlSettingsProvider.kt` to :core:common.
- Introducing `CssClassNameProvider`, `GlobalCssStyleProvider`, `PlainTextMessagePreElementCssStyleProvider`, and `SignatureCssStyleProvider` to dynamically generate CSS for message display.
- Updating `DisplayHtml`, `HtmlProcessor`, and their factories to use these new providers.
- Refactoring dependency injection to provide the new CSS services.
- Using CSS variables for blockquote border colors in `EmailTextToHtml`.
---
 .../api/css/DefaultCssClassNameProvider.kt    |   4 +-
 .../app/k9mail/DependencyInjectionTest.kt     |  13 +-
 .../android/DependencyInjectionTest.kt        |  13 +-
 cli/html-cleaner-cli/build.gradle.kts         |   2 +
 .../app/k9mail/cli/html/cleaner/Main.kt       |  26 +-
 .../inject/KoinMultibindingCollection.kt      |  19 ++
 .../core/common/mail}/html/HtmlSettings.kt    |   2 +-
 .../common/mail/html/HtmlSettingsProvider.kt  |   5 +
 .../mail/message/composer/build.gradle.kts    |   1 +
 .../MessageComposerHtmlSettingsProvider.kt    |  16 ++
 .../inject/FeatureMessageComposerModule.kt    |   2 +
 .../reader/api/css/CssStyleProvider.kt        |  18 +-
 .../html/MessageReaderHtmlSettingsProvider.kt |   5 +
 .../impl/css/DefaultGlobalCssStyleProvider.kt |  31 ++-
 ...inTextMessagePreElementCssStyleProvider.kt |   5 +-
 .../css/DefaultSignatureCssStyleProvider.kt   |   7 +
 ...efaultMessageReaderHtmlSettingsProvider.kt |  17 ++
 .../impl/inject/FeatureMessageReaderModule.kt |  17 +-
 legacy/core/build.gradle.kts                  |   1 +
 .../MessageViewInfoExtractorFactory.kt        |   2 +-
 .../com/fsck/k9/message/html/DisplayHtml.kt   |  76 +++---
 .../k9/message/html/DisplayHtmlFactory.kt     |  17 +-
 .../fsck/k9/message/html/EmailTextToHtml.kt   |  31 ++-
 .../k9/message/html/HtmlProcessorFactory.kt   |   7 +-
 .../com/fsck/k9/message/html/KoinModule.kt    |  16 +-
 .../core/src/test/java/com/fsck/k9/TestApp.kt |  14 ++
 .../fsck/k9/message/html/DisplayHtmlTest.kt   |  67 ++++-
 .../fsck/k9/message/html/HtmlConverterTest.kt | 228 +++++++++++-------
 .../java/com/fsck/k9/ui/base/KoinModule.kt    |   4 +-
 legacy/ui/legacy/build.gradle.kts             |   1 +
 .../java/com/fsck/k9/activity/KoinModule.kt   |   8 +-
 .../k9/activity/MessageLoaderHelperFactory.kt |  10 +-
 .../main/java/com/fsck/k9/ui/KoinModule.kt    |  12 +-
 .../fsck/k9/ui/helper/DisplayHtmlUiFactory.kt |  27 ++-
 .../fsck/k9/ui/helper/HtmlSettingsProvider.kt |  21 --
 .../src/test/java/com/fsck/k9/TestApp.kt      |   7 +
 library/html-cleaner/build.gradle.kts         |   2 +
 .../app/k9mail/html/cleaner/HtmlProcessor.kt  |  18 +-
 38 files changed, 549 insertions(+), 223 deletions(-)
 rename {legacy/core/src/main/java/com/fsck/k9/message => core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail}/html/HtmlSettings.kt (67%)
 create mode 100644 core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail/html/HtmlSettingsProvider.kt
 create mode 100644 feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/html/MessageComposerHtmlSettingsProvider.kt
 create mode 100644 feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/html/MessageReaderHtmlSettingsProvider.kt
 create mode 100644 feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/html/DefaultMessageReaderHtmlSettingsProvider.kt
 delete mode 100644 legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/HtmlSettingsProvider.kt

diff --git a/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt b/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
index 59d9bdf013b..45c41efe0cc 100644
--- a/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
+++ b/app-common/src/main/kotlin/net/thunderbird/android/feature/mail/message/reader/api/css/DefaultCssClassNameProvider.kt
@@ -13,9 +13,9 @@ class DefaultCssClassNameProvider(
     override val mainContentClassName: String = "${defaultNamespaceClassName}__main-content"
     override val plainTextMessagePreClassName: String =
         if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
-            EmailTextToHtml.K9MAIL_CSS_CLASS
-        } else {
             "${defaultNamespaceClassName}__plain-text-message-pre"
+        } else {
+            EmailTextToHtml.K9MAIL_CSS_CLASS
         }
     override val signatureClassName: String =
         if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
diff --git a/app-k9mail/src/test/kotlin/app/k9mail/DependencyInjectionTest.kt b/app-k9mail/src/test/kotlin/app/k9mail/DependencyInjectionTest.kt
index e813a0b02a1..9da93210e2b 100644
--- a/app-k9mail/src/test/kotlin/app/k9mail/DependencyInjectionTest.kt
+++ b/app-k9mail/src/test/kotlin/app/k9mail/DependencyInjectionTest.kt
@@ -15,15 +15,18 @@ import com.fsck.k9.job.MailSyncWorker
 import com.fsck.k9.job.SyncDebugWorker
 import com.fsck.k9.mailstore.AttachmentResolver
 import com.fsck.k9.message.html.DisplayHtml
-import com.fsck.k9.message.html.HtmlSettings
+import com.fsck.k9.message.html.DisplayHtmlFactory
 import com.fsck.k9.ui.changelog.ChangeLogMode
 import com.fsck.k9.ui.changelog.ChangelogViewModel
+import com.fsck.k9.ui.helper.DisplayHtmlUiFactory
 import com.fsck.k9.view.K9WebViewClient
 import com.fsck.k9.view.MessageWebView
 import net.openid.appauth.AppAuthConfiguration
+import net.thunderbird.core.common.mail.html.HtmlSettings
 import net.thunderbird.core.preference.storage.Storage
 import net.thunderbird.feature.account.AccountId
 import net.thunderbird.feature.mail.message.list.ui.dialog.SetupArchiveFolderDialogContract
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import org.junit.Test
 import org.koin.core.annotation.KoinExperimentalAPI
 import org.koin.test.verify.definition
@@ -53,7 +56,13 @@ class DependencyInjectionTest {
             injections = injectedParameters(
                 definition<AccountRemoverWorker>(WorkerParameters::class),
                 definition<ChangelogViewModel>(ChangeLogMode::class),
-                definition<DisplayHtml>(HtmlSettings::class),
+                definition<DisplayHtml>(
+                    HtmlSettings::class,
+                    CssClassNameProvider::class,
+                    List::class,
+                ),
+                definition<DisplayHtmlFactory>(List::class),
+                definition<DisplayHtmlUiFactory>(List::class),
                 definition<K9WebViewClient>(AttachmentResolver::class, MessageWebView.OnPageFinishedListener::class),
                 definition<MailSyncWorker>(WorkerParameters::class),
                 definition<SyncDebugWorker>(WorkerParameters::class),
diff --git a/app-thunderbird/src/test/kotlin/net/thunderbird/android/DependencyInjectionTest.kt b/app-thunderbird/src/test/kotlin/net/thunderbird/android/DependencyInjectionTest.kt
index ba388640f0b..1213d55afb2 100644
--- a/app-thunderbird/src/test/kotlin/net/thunderbird/android/DependencyInjectionTest.kt
+++ b/app-thunderbird/src/test/kotlin/net/thunderbird/android/DependencyInjectionTest.kt
@@ -15,15 +15,18 @@ import com.fsck.k9.job.MailSyncWorker
 import com.fsck.k9.job.SyncDebugWorker
 import com.fsck.k9.mailstore.AttachmentResolver
 import com.fsck.k9.message.html.DisplayHtml
-import com.fsck.k9.message.html.HtmlSettings
+import com.fsck.k9.message.html.DisplayHtmlFactory
 import com.fsck.k9.ui.changelog.ChangeLogMode
 import com.fsck.k9.ui.changelog.ChangelogViewModel
+import com.fsck.k9.ui.helper.DisplayHtmlUiFactory
 import com.fsck.k9.view.K9WebViewClient
 import com.fsck.k9.view.MessageWebView
 import net.openid.appauth.AppAuthConfiguration
+import net.thunderbird.core.common.mail.html.HtmlSettings
 import net.thunderbird.core.preference.storage.Storage
 import net.thunderbird.feature.account.AccountId
 import net.thunderbird.feature.mail.message.list.ui.dialog.SetupArchiveFolderDialogContract
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import org.junit.Test
 import org.koin.core.annotation.KoinExperimentalAPI
 import org.koin.test.verify.definition
@@ -53,7 +56,13 @@ class DependencyInjectionTest {
             injections = injectedParameters(
                 definition<AccountRemoverWorker>(WorkerParameters::class),
                 definition<ChangelogViewModel>(ChangeLogMode::class),
-                definition<DisplayHtml>(HtmlSettings::class),
+                definition<DisplayHtml>(
+                    HtmlSettings::class,
+                    CssClassNameProvider::class,
+                    List::class,
+                ),
+                definition<DisplayHtmlFactory>(List::class),
+                definition<DisplayHtmlUiFactory>(List::class),
                 definition<K9WebViewClient>(AttachmentResolver::class, MessageWebView.OnPageFinishedListener::class),
                 definition<MailSyncWorker>(WorkerParameters::class),
                 definition<SyncDebugWorker>(WorkerParameters::class),
diff --git a/cli/html-cleaner-cli/build.gradle.kts b/cli/html-cleaner-cli/build.gradle.kts
index 9f8b4e20d93..6f682f5adff 100644
--- a/cli/html-cleaner-cli/build.gradle.kts
+++ b/cli/html-cleaner-cli/build.gradle.kts
@@ -9,6 +9,8 @@ application {
 }
 
 dependencies {
+    implementation(projects.core.featureflag)
+    implementation(projects.feature.mail.message.reader.api)
     implementation(projects.library.htmlCleaner)
 
     implementation(libs.clikt)
diff --git a/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt b/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt
index 79cf382d7fe..608ccbe3469 100644
--- a/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt
+++ b/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt
@@ -6,10 +6,14 @@ import com.github.ajalt.clikt.core.CliktCommand
 import com.github.ajalt.clikt.core.Context
 import com.github.ajalt.clikt.core.main
 import com.github.ajalt.clikt.parameters.arguments.argument
+import com.github.ajalt.clikt.parameters.arguments.default
 import com.github.ajalt.clikt.parameters.arguments.optional
+import com.github.ajalt.clikt.parameters.types.enum
 import com.github.ajalt.clikt.parameters.types.file
 import com.github.ajalt.clikt.parameters.types.inputStream
 import java.io.File
+import net.thunderbird.core.featureflag.FeatureFlagResult
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import okio.buffer
 import okio.sink
 import okio.source
@@ -19,6 +23,10 @@ class HtmlCleaner : CliktCommand() {
     val input by argument(help = "HTML input file (needs to be UTF-8 encoded)")
         .inputStream()
 
+    val app by argument(help = "The app the email will be target to")
+        .enum<App>(ignoreCase = true)
+        .default(App.THUNDERBIRD)
+
     val output by argument(help = "Output file")
         .file(mustExist = false, canBeDir = false)
         .optional()
@@ -38,7 +46,21 @@ class HtmlCleaner : CliktCommand() {
 
     private fun cleanHtml(html: String): String {
         val htmlProcessor = HtmlProcessor(
-            object : HtmlHeadProvider {
+            featureFlagProvider = { FeatureFlagResult.Enabled },
+            cssClassNameProvider = object : CssClassNameProvider {
+                override val defaultNamespaceClassName: String
+                    get() = if (app == App.THUNDERBIRD) {
+                        "net_thunderbird_android"
+                    } else {
+                        "com_fsck_k9"
+                    }
+                override val rootClassName: String = "${defaultNamespaceClassName}__message-viewer"
+                override val mainContentClassName: String = "${defaultNamespaceClassName}__main-content"
+                override val plainTextMessagePreClassName: String =
+                    "${defaultNamespaceClassName}__plain-text-message-pre"
+                override val signatureClassName: String = "${defaultNamespaceClassName}__signature"
+            },
+            htmlHeadProvider = object : HtmlHeadProvider {
                 override val headHtml = """<meta name="viewport" content="width=device-width"/>"""
             },
         )
@@ -57,4 +79,6 @@ class HtmlCleaner : CliktCommand() {
     }
 }
 
+enum class App { THUNDERBIRD, K9MAIL }
+
 fun main(args: Array<String>) = HtmlCleaner().main(args)
diff --git a/core/common/src/commonMain/kotlin/net/thunderbird/core/common/inject/KoinMultibindingCollection.kt b/core/common/src/commonMain/kotlin/net/thunderbird/core/common/inject/KoinMultibindingCollection.kt
index 4263f311885..f75207b992c 100644
--- a/core/common/src/commonMain/kotlin/net/thunderbird/core/common/inject/KoinMultibindingCollection.kt
+++ b/core/common/src/commonMain/kotlin/net/thunderbird/core/common/inject/KoinMultibindingCollection.kt
@@ -29,6 +29,25 @@ inline fun <reified T> Module.singleListOf(vararg items: Definition<T>, qualifie
     }
 }
 
+/**
+ * Defines a factory for a list of elements of type [T].
+ *
+ * This function creates a factory definition for a list of elements. Each time this list is
+ * requested, a new list is created, and each element in the list is resolved anew from the
+ * provided [items] definitions.
+ *
+ * @param T The type of elements in the list.
+ * @param items Vararg of [Definition]s that will be resolved and added to the list.
+ * @param qualifier Optional [Qualifier] to distinguish this list from others of the same type.
+ *                  If null, a default qualifier based on the type [T] will be used.
+ */
+@KoinDslMarker
+inline fun <reified T> Module.factoryListOf(vararg items: Definition<T>, qualifier: Qualifier? = null) {
+    factory(qualifier ?: defaultListQualifier<T>()) {
+        items.map { definition -> definition(this, parametersOf()) }
+    }
+}
+
 /**
  * Resolves a [List] of instances of type [T].
  * This is a helper function for Koin's multibinding feature.
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlSettings.kt b/core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail/html/HtmlSettings.kt
similarity index 67%
rename from legacy/core/src/main/java/com/fsck/k9/message/html/HtmlSettings.kt
rename to core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail/html/HtmlSettings.kt
index 0eb934e4bb9..3fd71b5021b 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlSettings.kt
+++ b/core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail/html/HtmlSettings.kt
@@ -1,4 +1,4 @@
-package com.fsck.k9.message.html
+package net.thunderbird.core.common.mail.html
 
 data class HtmlSettings(
     val useDarkMode: Boolean,
diff --git a/core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail/html/HtmlSettingsProvider.kt b/core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail/html/HtmlSettingsProvider.kt
new file mode 100644
index 00000000000..0f9ee2b5dd4
--- /dev/null
+++ b/core/common/src/commonMain/kotlin/net/thunderbird/core/common/mail/html/HtmlSettingsProvider.kt
@@ -0,0 +1,5 @@
+package net.thunderbird.core.common.mail.html
+
+fun interface HtmlSettingsProvider {
+    fun create(): HtmlSettings
+}
diff --git a/feature/mail/message/composer/build.gradle.kts b/feature/mail/message/composer/build.gradle.kts
index c80bbe6a3e1..90067d40f7b 100644
--- a/feature/mail/message/composer/build.gradle.kts
+++ b/feature/mail/message/composer/build.gradle.kts
@@ -8,6 +8,7 @@ android {
 }
 
 dependencies {
+    implementation(projects.core.common)
     implementation(projects.core.ui.compose.designsystem)
     implementation(projects.core.ui.theme.api)
     implementation(projects.feature.notification.api)
diff --git a/feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/html/MessageComposerHtmlSettingsProvider.kt b/feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/html/MessageComposerHtmlSettingsProvider.kt
new file mode 100644
index 00000000000..71199e9b5c1
--- /dev/null
+++ b/feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/html/MessageComposerHtmlSettingsProvider.kt
@@ -0,0 +1,16 @@
+package net.thunderbird.feature.mail.message.composer.html
+
+import net.thunderbird.core.common.mail.html.HtmlSettings
+import net.thunderbird.core.common.mail.html.HtmlSettingsProvider
+import net.thunderbird.core.ui.theme.api.Theme
+import net.thunderbird.core.ui.theme.api.ThemeManager
+
+interface MessageComposerHtmlSettingsProvider : HtmlSettingsProvider
+
+internal fun MessageComposerHtmlSettingsProvider(themeManager: ThemeManager): MessageComposerHtmlSettingsProvider =
+    object : MessageComposerHtmlSettingsProvider {
+        override fun create(): HtmlSettings = HtmlSettings(
+            useDarkMode = themeManager.messageComposeTheme == Theme.DARK,
+            useFixedWidthFont = false,
+        )
+    }
diff --git a/feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/inject/FeatureMessageComposerModule.kt b/feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/inject/FeatureMessageComposerModule.kt
index 38ed1056004..ec944789b36 100644
--- a/feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/inject/FeatureMessageComposerModule.kt
+++ b/feature/mail/message/composer/src/main/kotlin/net/thunderbird/feature/mail/message/composer/inject/FeatureMessageComposerModule.kt
@@ -2,10 +2,12 @@ package net.thunderbird.feature.mail.message.composer.inject
 
 import net.thunderbird.feature.mail.message.composer.dialog.SentFolderNotFoundConfirmationDialogFragment
 import net.thunderbird.feature.mail.message.composer.dialog.SentFolderNotFoundConfirmationDialogFragmentFactory
+import net.thunderbird.feature.mail.message.composer.html.MessageComposerHtmlSettingsProvider
 import org.koin.dsl.module
 
 val featureMessageComposerModule = module {
     factory<SentFolderNotFoundConfirmationDialogFragmentFactory> {
         SentFolderNotFoundConfirmationDialogFragment.Factory
     }
+    factory<MessageComposerHtmlSettingsProvider> { MessageComposerHtmlSettingsProvider(themeManager = get()) }
 }
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt
index 8ec618aa1cf..098e65bf883 100644
--- a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt
+++ b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/css/CssStyleProvider.kt
@@ -1,5 +1,7 @@
 package net.thunderbird.feature.mail.message.reader.api.css
 
+import net.thunderbird.core.common.mail.html.HtmlSettings
+
 /**
  * Provides CSS styles to be injected into the message viewer's web view.
  *
@@ -19,6 +21,10 @@ sealed interface CssStyleProvider {
      * ```
      */
     val style: String
+
+    fun interface Factory {
+        fun create(htmlSettings: HtmlSettings): CssStyleProvider
+    }
 }
 
 /**
@@ -27,7 +33,9 @@ sealed interface CssStyleProvider {
  * This is used for global styles that affect the overall appearance of the email content,
  * such as font size, colors, and layout adjustments for the entire message body.
  */
-interface GlobalCssStyleProvider : CssStyleProvider
+interface GlobalCssStyleProvider : CssStyleProvider {
+    fun interface Factory : CssStyleProvider.Factory
+}
 
 /**
  * Provides CSS styles specifically for the `<pre>` element used to wrap plain text messages.
@@ -36,9 +44,7 @@ interface GlobalCssStyleProvider : CssStyleProvider
  * size, and controlling word wrapping behavior to ensure readability.
  */
 interface PlainTextMessagePreElementCssStyleProvider : CssStyleProvider {
-    interface Factory {
-        fun create(useFixedWidthFont: Boolean): PlainTextMessagePreElementCssStyleProvider
-    }
+    fun interface Factory : CssStyleProvider.Factory
 }
 
 /**
@@ -47,4 +53,6 @@ interface PlainTextMessagePreElementCssStyleProvider : CssStyleProvider {
  * This is used to apply custom styling to the signature block within an email message,
  * allowing it to be visually distinct from the main message body.
  */
-interface SignatureCssStyleProvider : CssStyleProvider
+interface SignatureCssStyleProvider : CssStyleProvider {
+    fun interface Factory : CssStyleProvider.Factory
+}
diff --git a/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/html/MessageReaderHtmlSettingsProvider.kt b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/html/MessageReaderHtmlSettingsProvider.kt
new file mode 100644
index 00000000000..eb4e247b740
--- /dev/null
+++ b/feature/mail/message/reader/api/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/api/html/MessageReaderHtmlSettingsProvider.kt
@@ -0,0 +1,5 @@
+package net.thunderbird.feature.mail.message.reader.api.html
+
+import net.thunderbird.core.common.mail.html.HtmlSettingsProvider
+
+interface MessageReaderHtmlSettingsProvider : HtmlSettingsProvider
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
index 1768e910d24..49de988d992 100644
--- a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt
@@ -1,11 +1,13 @@
 package net.thunderbird.feature.mail.message.reader.impl.css
 
+import net.thunderbird.core.common.mail.html.HtmlSettings
 import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssStyleProvider
 import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
 import net.thunderbird.feature.mail.message.reader.api.css.GlobalCssStyleProvider
 import org.intellij.lang.annotations.Language
 
-internal class DefaultGlobalCssStyleProvider(
+internal class DefaultGlobalCssStyleProvider private constructor(
     cssClassNameProvider: CssClassNameProvider,
     cssVariableNameProvider: CssVariableNameProvider,
 ) : GlobalCssStyleProvider {
@@ -26,6 +28,7 @@ internal class DefaultGlobalCssStyleProvider(
         |  .${cssClassNameProvider.rootClassName}.${cssClassNameProvider.mainContentClassName} {
         |    width: 100%;
         |    overflow-wrap: break-word;
+        |    padding: 0 8px;
         |  }
         |  .${cssClassNameProvider.rootClassName}.${cssClassNameProvider.mainContentClassName} pre {
         |    white-space: pre-wrap;
@@ -39,11 +42,22 @@ internal class DefaultGlobalCssStyleProvider(
         |  }
         |</style>
     """.trimMargin()
+
+    internal class Factory(
+        private val cssClassNameProvider: CssClassNameProvider,
+        private val cssVariableNameProvider: CssVariableNameProvider,
+    ) : GlobalCssStyleProvider.Factory {
+        override fun create(htmlSettings: HtmlSettings): CssStyleProvider = DefaultGlobalCssStyleProvider(
+            cssClassNameProvider = cssClassNameProvider,
+            cssVariableNameProvider = cssVariableNameProvider,
+        )
+    }
 }
 
-internal class LegacyGlobalCssStyleProvider : GlobalCssStyleProvider {
+internal class LegacyGlobalCssStyleProvider(useDarkMode: Boolean) : GlobalCssStyleProvider {
     @Language("HTML")
-    override val style: String = """
+    override val style: String = if (useDarkMode) {
+        """
         |<style type="text/css">
         |  * {
         |    background: #121212 !important;
@@ -52,5 +66,14 @@ internal class LegacyGlobalCssStyleProvider : GlobalCssStyleProvider {
         |  :link, :link * { color: #CCFF33 !important }
         |  :visited, :visited * { color: #551A8B !important }
         |</style>
-    """.trimMargin()
+        """.trimMargin()
+    } else {
+        ""
+    }
+
+    internal class Factory : GlobalCssStyleProvider.Factory {
+        override fun create(htmlSettings: HtmlSettings): CssStyleProvider = LegacyGlobalCssStyleProvider(
+            useDarkMode = htmlSettings.useDarkMode,
+        )
+    }
 }
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt
index 2f15a167016..69e6f852621 100644
--- a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultPlainTextMessagePreElementCssStyleProvider.kt
@@ -1,5 +1,6 @@
 package net.thunderbird.feature.mail.message.reader.impl.css
 
+import net.thunderbird.core.common.mail.html.HtmlSettings
 import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import net.thunderbird.feature.mail.message.reader.api.css.PlainTextMessagePreElementCssStyleProvider
 import org.intellij.lang.annotations.Language
@@ -24,10 +25,10 @@ class DefaultPlainTextMessagePreElementCssStyleProvider(
     class Factory(
         private val cssClassNameProvider: CssClassNameProvider,
     ) : PlainTextMessagePreElementCssStyleProvider.Factory {
-        override fun create(useFixedWidthFont: Boolean): PlainTextMessagePreElementCssStyleProvider {
+        override fun create(htmlSettings: HtmlSettings): PlainTextMessagePreElementCssStyleProvider {
             return DefaultPlainTextMessagePreElementCssStyleProvider(
                 cssClassNameProvider = cssClassNameProvider,
-                useFixedWidthFont = useFixedWidthFont,
+                useFixedWidthFont = htmlSettings.useFixedWidthFont,
             )
         }
     }
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt
index 1b959894da3..a2dadab5ccb 100644
--- a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultSignatureCssStyleProvider.kt
@@ -1,6 +1,8 @@
 package net.thunderbird.feature.mail.message.reader.impl.css
 
+import net.thunderbird.core.common.mail.html.HtmlSettings
 import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssStyleProvider
 import net.thunderbird.feature.mail.message.reader.api.css.SignatureCssStyleProvider
 import org.intellij.lang.annotations.Language
 
@@ -15,4 +17,9 @@ class DefaultSignatureCssStyleProvider(
         |  }
         |</style>
     """.trimMargin()
+
+    class Factory(private val cssClassNameProvider: CssClassNameProvider) : SignatureCssStyleProvider.Factory {
+        override fun create(htmlSettings: HtmlSettings): CssStyleProvider =
+            DefaultSignatureCssStyleProvider(cssClassNameProvider)
+    }
 }
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/html/DefaultMessageReaderHtmlSettingsProvider.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/html/DefaultMessageReaderHtmlSettingsProvider.kt
new file mode 100644
index 00000000000..1d2518b100b
--- /dev/null
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/html/DefaultMessageReaderHtmlSettingsProvider.kt
@@ -0,0 +1,17 @@
+package net.thunderbird.feature.mail.message.reader.impl.html
+
+import net.thunderbird.core.common.mail.html.HtmlSettings
+import net.thunderbird.core.preference.display.visualSettings.DisplayVisualSettingsPreferenceManager
+import net.thunderbird.core.ui.theme.api.Theme
+import net.thunderbird.core.ui.theme.api.ThemeManager
+import net.thunderbird.feature.mail.message.reader.api.html.MessageReaderHtmlSettingsProvider
+
+internal class DefaultMessageReaderHtmlSettingsProvider(
+    private val themeManager: ThemeManager,
+    private val visualSettingsPreferenceManager: DisplayVisualSettingsPreferenceManager,
+) : MessageReaderHtmlSettingsProvider {
+    override fun create(): HtmlSettings = HtmlSettings(
+        useDarkMode = themeManager.messageViewTheme == Theme.DARK,
+        useFixedWidthFont = visualSettingsPreferenceManager.getConfig().isUseMessageViewFixedWidthFont,
+    )
+}
diff --git a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
index 7b9d5dbf7d3..10d7044e788 100644
--- a/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
+++ b/feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/inject/FeatureMessageReaderModule.kt
@@ -1,33 +1,42 @@
 package net.thunderbird.feature.mail.message.reader.impl.inject
 
+import net.thunderbird.core.common.inject.factoryListOf
 import net.thunderbird.core.featureflag.FeatureFlagProvider
 import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
 import net.thunderbird.feature.mail.message.reader.api.css.GlobalCssStyleProvider
 import net.thunderbird.feature.mail.message.reader.api.css.PlainTextMessagePreElementCssStyleProvider
 import net.thunderbird.feature.mail.message.reader.api.css.SignatureCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.html.MessageReaderHtmlSettingsProvider
 import net.thunderbird.feature.mail.message.reader.impl.css.DefaultCssVariableNameProvider
 import net.thunderbird.feature.mail.message.reader.impl.css.DefaultGlobalCssStyleProvider
 import net.thunderbird.feature.mail.message.reader.impl.css.DefaultPlainTextMessagePreElementCssStyleProvider
 import net.thunderbird.feature.mail.message.reader.impl.css.DefaultSignatureCssStyleProvider
 import net.thunderbird.feature.mail.message.reader.impl.css.LegacyGlobalCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.impl.html.DefaultMessageReaderHtmlSettingsProvider
 import org.koin.dsl.module
 
 val featureMessageReaderModule = module {
+    factory<MessageReaderHtmlSettingsProvider> { DefaultMessageReaderHtmlSettingsProvider(get(), get()) }
     single<CssVariableNameProvider> { DefaultCssVariableNameProvider(get()) }
-    single<GlobalCssStyleProvider> {
+    factory<GlobalCssStyleProvider.Factory> {
         val featureFlagProvider = get<FeatureFlagProvider>()
         if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
-            DefaultGlobalCssStyleProvider(
+            DefaultGlobalCssStyleProvider.Factory(
                 cssClassNameProvider = get(),
                 cssVariableNameProvider = get(),
             )
         } else {
-            LegacyGlobalCssStyleProvider()
+            LegacyGlobalCssStyleProvider.Factory()
         }
     }
-    single<SignatureCssStyleProvider> { DefaultSignatureCssStyleProvider(get()) }
+    factory<SignatureCssStyleProvider.Factory> { DefaultSignatureCssStyleProvider.Factory(get()) }
     factory<PlainTextMessagePreElementCssStyleProvider.Factory> {
         DefaultPlainTextMessagePreElementCssStyleProvider.Factory(cssClassNameProvider = get())
     }
+    factoryListOf(
+        { get<GlobalCssStyleProvider.Factory>() },
+        { get<SignatureCssStyleProvider.Factory>() },
+        { get<PlainTextMessagePreElementCssStyleProvider.Factory>() },
+    )
 }
diff --git a/legacy/core/build.gradle.kts b/legacy/core/build.gradle.kts
index 92c06fc4b7e..b4a275d0f33 100644
--- a/legacy/core/build.gradle.kts
+++ b/legacy/core/build.gradle.kts
@@ -47,6 +47,7 @@ dependencies {
     implementation(libs.mime4j.dom)
     implementation(libs.uri)
     implementation(projects.feature.navigation.drawer.api)
+    implementation(projects.feature.mail.message.reader.api)
 
     testApi(projects.core.testing)
     testApi(projects.core.android.testing)
diff --git a/legacy/core/src/main/java/com/fsck/k9/mailstore/MessageViewInfoExtractorFactory.kt b/legacy/core/src/main/java/com/fsck/k9/mailstore/MessageViewInfoExtractorFactory.kt
index 98194d8cf75..e40a5f069fa 100644
--- a/legacy/core/src/main/java/com/fsck/k9/mailstore/MessageViewInfoExtractorFactory.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/mailstore/MessageViewInfoExtractorFactory.kt
@@ -3,7 +3,7 @@ package com.fsck.k9.mailstore
 import com.fsck.k9.CoreResourceProvider
 import com.fsck.k9.message.extractors.AttachmentInfoExtractor
 import com.fsck.k9.message.html.HtmlProcessorFactory
-import com.fsck.k9.message.html.HtmlSettings
+import net.thunderbird.core.common.mail.html.HtmlSettings
 
 class MessageViewInfoExtractorFactory(
     private val attachmentInfoExtractor: AttachmentInfoExtractor,
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtml.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtml.kt
index f1551b3aa2d..07583970857 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtml.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtml.kt
@@ -1,59 +1,43 @@
 package com.fsck.k9.message.html
 
 import app.k9mail.html.cleaner.HtmlHeadProvider
-
-class DisplayHtml(private val settings: HtmlSettings) : HtmlHeadProvider {
+import net.thunderbird.core.common.mail.html.HtmlSettings
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssStyleProvider
+import org.intellij.lang.annotations.Language
+
+class DisplayHtml(
+    private val htmlSettings: HtmlSettings,
+    private val cssClassNameProvider: CssClassNameProvider,
+    private val cssStyleProviders: List<CssStyleProvider.Factory>,
+) : HtmlHeadProvider {
     override val headHtml: String
-        get() {
-            return """<meta name="viewport" content="width=device-width"/>""" +
-                cssStyleTheme() +
-                cssStylePre() +
-                cssStyleSignature()
-        }
+        @Language("HTML")
+        get() = """
+            |<meta name="viewport" content="width=device-width"/>
+            |${cssStyleProviders.joinToString("\n") { it.create(htmlSettings).style }}
+        """.trimMargin()
 
     fun wrapStatusMessage(status: CharSequence): String {
         return wrapMessageContent("<div style=\"text-align:center; color: grey;\">$status</div>")
     }
 
+    @Language("HTML")
     fun wrapMessageContent(messageContent: CharSequence): String {
+        // TODO(#10074): This should be consolidated in a single place. There are many places that
+        //               build an HTML document to almost the same purpose (Viewer, Composer, etc).
+        //               Current files that build the HTML document: this, HtmlProcessor.kt, and
+        //               HtmlQuoteCreator.java (maybe).
         // Include a meta tag so the WebView will not use a fixed viewport width of 980 px
-        return "<html dir=\"auto\"><head><meta name=\"viewport\" content=\"width=device-width\"/>" +
-            cssStyleTheme() +
-            cssStylePre() +
-            "</head><body>" +
-            messageContent +
-            "</body></html>"
-    }
-
-    private fun cssStyleTheme(): String {
-        return if (settings.useDarkMode) {
-            // TODO: Don't hardcode these values. Inject them via HtmlSettings.
-            "<style type=\"text/css\">" +
-                "* { background: #121212 ! important; color: #F3F3F3 !important }" +
-                ":link, :link * { color: #CCFF33 !important }" +
-                ":visited, :visited * { color: #551A8B !important }</style> "
-        } else {
-            ""
-        }
-    }
-
-    /**
-     * Dynamically generate a CSS style for `<pre>` elements.
-     *
-     * The style incorporates the user's current preference setting for the font family used for plain text messages.
-     *
-     * @return A `<style>` element that can be dynamically included in the HTML `<head>` element when messages are
-     * displayed.
-     */
-    private fun cssStylePre(): String {
-        val font = if (settings.useFixedWidthFont) "monospace" else "sans-serif"
-
-        return "<style type=\"text/css\"> pre." + EmailTextToHtml.K9MAIL_CSS_CLASS +
-            " {white-space: pre-wrap; word-wrap:break-word; " +
-            "font-family: " + font + "; margin-top: 0px}</style>"
-    }
-
-    private fun cssStyleSignature(): String {
-        return """<style type="text/css">.k9mail-signature { opacity: 0.5 }</style>"""
+        return """
+            |<html dir="auto">
+            |  <head>
+            |    $headHtml
+            |  </head>
+            |  <body class="${cssClassNameProvider.rootClassName} ${cssClassNameProvider.mainContentClassName}">
+            |    $messageContent
+            |  </body>
+            |</html>
+        """.trimMargin()
     }
 }
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtmlFactory.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtmlFactory.kt
index 8e717af9d74..880505d2eef 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtmlFactory.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/DisplayHtmlFactory.kt
@@ -1,7 +1,16 @@
 package com.fsck.k9.message.html
 
-class DisplayHtmlFactory {
-    fun create(settings: HtmlSettings): DisplayHtml {
-        return DisplayHtml(settings)
-    }
+import net.thunderbird.core.common.mail.html.HtmlSettings
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssStyleProvider
+
+class DisplayHtmlFactory(
+    private val cssClassNameProvider: CssClassNameProvider,
+    private val cssStyleProviders: List<CssStyleProvider.Factory>,
+) {
+    fun create(settings: HtmlSettings): DisplayHtml = DisplayHtml(
+        htmlSettings = settings,
+        cssClassNameProvider = cssClassNameProvider,
+        cssStyleProviders = cssStyleProviders,
+    )
 }
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/EmailTextToHtml.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/EmailTextToHtml.kt
index 81c4e2db35d..3c6bc86064c 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/EmailTextToHtml.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/EmailTextToHtml.kt
@@ -1,6 +1,15 @@
 package com.fsck.k9.message.html
 
-class EmailTextToHtml private constructor(private val text: String) {
+import app.k9mail.legacy.di.DI
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
+import org.intellij.lang.annotations.Language
+
+class EmailTextToHtml private constructor(
+    private val cssClassNameProvider: CssClassNameProvider,
+    private val cssVariableNameProvider: CssVariableNameProvider,
+    private val text: String,
+) {
     private val html = StringBuilder(text.length + EXTRA_BUFFER_LENGTH)
     private var previousQuoteDepth = 0
 
@@ -22,7 +31,7 @@ class EmailTextToHtml private constructor(private val text: String) {
     }
 
     private fun appendHtmlPrefix() {
-        html.append("<pre class=\"$K9MAIL_CSS_CLASS\">")
+        html.append("<pre class=\"${cssClassNameProvider.plainTextMessagePreClassName}\">")
     }
 
     private fun appendHtmlSuffix() {
@@ -36,13 +45,13 @@ class EmailTextToHtml private constructor(private val text: String) {
             }
         } else if (quoteDepth > previousQuoteDepth) {
             for (depth in (previousQuoteDepth + 1)..quoteDepth) {
-                html.append(
-                    "<blockquote " +
-                        "class=\"gmail_quote\" " +
-                        "style=\"margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid ",
-                )
-                html.append(quoteColor(depth))
-                html.append("; padding-left: 1ex;\">")
+                val borderColorStyle =
+                    "${cssVariableNameProvider.blockquoteDefaultBorderLeftColor}: ${quoteColor(depth)};"
+
+                @Language("HTML")
+                val blockQuoteStartTag =
+                    "<blockquote class=\"gmail_quote\" style=\"margin-bottom: 1ex; $borderColorStyle\">"
+                html.append(blockQuoteStartTag)
             }
         }
         previousQuoteDepth = quoteDepth
@@ -63,7 +72,9 @@ class EmailTextToHtml private constructor(private val text: String) {
 
         @JvmStatic
         fun convert(text: String): String {
-            return EmailTextToHtml(text).convert()
+            val cssClassNameProvider = DI.get<CssClassNameProvider>()
+            val cssVariableNameProvider = DI.get<CssVariableNameProvider>()
+            return EmailTextToHtml(cssClassNameProvider, cssVariableNameProvider, text).convert()
         }
     }
 }
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
index 6ea3392421d..06eaadfebd0 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
@@ -1,12 +1,17 @@
 package com.fsck.k9.message.html
 
 import app.k9mail.html.cleaner.HtmlProcessor
+import net.thunderbird.core.common.mail.html.HtmlSettings
+import net.thunderbird.core.featureflag.FeatureFlagProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 
 class HtmlProcessorFactory(
+    private val featureFlagProvider: FeatureFlagProvider,
+    private val cssClassNameProvider: CssClassNameProvider,
     private val displayHtmlFactory: DisplayHtmlFactory,
 ) {
     fun create(settings: HtmlSettings): HtmlProcessor {
         val displayHtml = displayHtmlFactory.create(settings)
-        return HtmlProcessor(displayHtml)
+        return HtmlProcessor(featureFlagProvider, cssClassNameProvider, displayHtml)
     }
 }
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/KoinModule.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/KoinModule.kt
index 7c6151ad0c8..74ca64e30ba 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/KoinModule.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/KoinModule.kt
@@ -1,8 +1,20 @@
 package com.fsck.k9.message.html
 
+import net.thunderbird.core.common.inject.getList
 import org.koin.dsl.module
 
 val htmlModule = module {
-    single { HtmlProcessorFactory(displayHtmlFactory = get()) }
-    single { DisplayHtmlFactory() }
+    factory {
+        HtmlProcessorFactory(
+            featureFlagProvider = get(),
+            cssClassNameProvider = get(),
+            displayHtmlFactory = get(),
+        )
+    }
+    factory {
+        DisplayHtmlFactory(
+            cssClassNameProvider = get(),
+            cssStyleProviders = getList(),
+        )
+    }
 }
diff --git a/legacy/core/src/test/java/com/fsck/k9/TestApp.kt b/legacy/core/src/test/java/com/fsck/k9/TestApp.kt
index 555d2c8f836..b19df888228 100644
--- a/legacy/core/src/test/java/com/fsck/k9/TestApp.kt
+++ b/legacy/core/src/test/java/com/fsck/k9/TestApp.kt
@@ -17,6 +17,7 @@ import net.thunderbird.core.android.account.AccountDefaultsProvider
 import net.thunderbird.core.android.account.LegacyAccountManager
 import net.thunderbird.core.android.preferences.TestStoragePersister
 import net.thunderbird.core.common.appConfig.PlatformConfigProvider
+import net.thunderbird.core.common.inject.factoryListOf
 import net.thunderbird.core.featureflag.FeatureFlag
 import net.thunderbird.core.featureflag.FeatureFlagProvider
 import net.thunderbird.core.featureflag.InMemoryFeatureFlagProvider
@@ -32,12 +33,17 @@ import net.thunderbird.core.logging.testing.TestLogLevelManager
 import net.thunderbird.core.logging.testing.TestLogger
 import net.thunderbird.core.preference.storage.StoragePersister
 import net.thunderbird.feature.mail.folder.api.OutboxFolderManager
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
 import net.thunderbird.legacy.core.FakeAccountDefaultsProvider
 import net.thunderbird.legacy.core.mailstore.folder.FakeOutboxFolderManager
 import org.koin.core.qualifier.named
 import org.koin.dsl.bind
 import org.koin.dsl.module
+import org.mockito.kotlin.doReturn
 import org.mockito.kotlin.mock
+import org.mockito.kotlin.whenever
 
 class TestApp : Application() {
     override fun onCreate() {
@@ -98,6 +104,14 @@ val testModule = module {
     single<LegacyAccountManager> { mock() }
     single<NotificationIconResourceProvider> { TestNotificationIconResourceProvider() }
     single<PlatformConfigProvider> { FakePlatformConfigProvider() }
+    single<CssVariableNameProvider> { mock() }
+    single<CssClassNameProvider> {
+        mock {
+            whenever(it.plainTextMessagePreClassName).doReturn("k9mail")
+        }
+    }
+    factoryListOf<CssStyleProvider>()
+    single<NotificationResourceProvider> { mock() }
 }
 
 class FakePlatformConfigProvider : PlatformConfigProvider {
diff --git a/legacy/core/src/test/java/com/fsck/k9/message/html/DisplayHtmlTest.kt b/legacy/core/src/test/java/com/fsck/k9/message/html/DisplayHtmlTest.kt
index 41132e96c85..fd16c62ed1f 100644
--- a/legacy/core/src/test/java/com/fsck/k9/message/html/DisplayHtmlTest.kt
+++ b/legacy/core/src/test/java/com/fsck/k9/message/html/DisplayHtmlTest.kt
@@ -2,13 +2,25 @@ package com.fsck.k9.message.html
 
 import assertk.Assert
 import assertk.assertThat
+import assertk.assertions.atLeast
 import assertk.assertions.hasSize
 import assertk.assertions.isEqualTo
+import assertk.assertions.prop
+import net.thunderbird.core.common.mail.html.HtmlSettings
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.GlobalCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.css.PlainTextMessagePreElementCssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.css.SignatureCssStyleProvider
 import org.jsoup.Jsoup
+import org.jsoup.nodes.Element
 import org.junit.Test
 
 class DisplayHtmlTest {
-    val displayHtml = DisplayHtml(HtmlSettings(useDarkMode = false, useFixedWidthFont = false))
+    val displayHtml = DisplayHtml(
+        htmlSettings = HtmlSettings(useDarkMode = false, useFixedWidthFont = false),
+        cssClassNameProvider = FakeCssClassNameProvider(),
+        cssStyleProviders = listOf(),
+    )
 
     @Test
     fun wrapMessageContent_addsViewportMetaElement() {
@@ -26,18 +38,31 @@ class DisplayHtmlTest {
 
     @Test
     fun wrapMessageContent_addsPreCSS() {
+        val expectedStyleCssRule = ".my-custom-pre-class { mock-property: mock-value }"
+        val displayHtml = DisplayHtml(
+            htmlSettings = HtmlSettings(useDarkMode = false, useFixedWidthFont = false),
+            cssClassNameProvider = FakeCssClassNameProvider(),
+            cssStyleProviders = listOf(
+                GlobalCssStyleProvider.Factory { FakeGlobalCssStyleProvider() },
+                PlainTextMessagePreElementCssStyleProvider.Factory {
+                    FakePlainTextMessagePreElementCssStyleProvider(
+                        style = "<style>$expectedStyleCssRule</style>",
+                    )
+                },
+                SignatureCssStyleProvider.Factory { FakeSignatureCssStyleProvider() },
+            ),
+        )
         val html = displayHtml.wrapMessageContent("Some text")
 
-        assertThat(html).containsHtmlElement("head > style")
-    }
-
-    @Test
-    fun wrapMessageContent_whenDarkMessageViewTheme_addsDarkThemeCSS() {
-        val darkModeDisplayHtml = DisplayHtml(HtmlSettings(useDarkMode = true, useFixedWidthFont = false))
-
-        val html = darkModeDisplayHtml.wrapMessageContent("Some text")
-
-        assertThat(html).htmlElements("head > style").hasSize(2)
+        assertThat(html).given { raw ->
+            val html = Jsoup.parse(raw)
+            assertThat(html.select("head > style"))
+                .atLeast(1) { element ->
+                    element
+                        .prop(Element::data)
+                        .isEqualTo(expectedStyleCssRule)
+                }
+        }
     }
 
     @Test
@@ -60,4 +85,24 @@ class DisplayHtmlTest {
     private fun Assert<String>.bodyText() = transform { html ->
         Jsoup.parse(html).body().text()
     }
+
+    private class FakeCssClassNameProvider(
+        override val defaultNamespaceClassName: String = "mock defaultNamespaceClassName",
+        override val rootClassName: String = "mock rootClassName",
+        override val mainContentClassName: String = "mock mainContentClassName",
+        override val plainTextMessagePreClassName: String = "mock plainTextMessagePreClassName",
+        override val signatureClassName: String = "mock signatureClassName",
+    ) : CssClassNameProvider
+
+    private class FakeGlobalCssStyleProvider(
+        override val style: String = "<style>.mock-style { mock-property: mock-value }</style>",
+    ) : GlobalCssStyleProvider
+
+    private class FakePlainTextMessagePreElementCssStyleProvider(
+        override val style: String = "<style>.mock-style { mock-property: mock-value }</style>",
+    ) : PlainTextMessagePreElementCssStyleProvider
+
+    private class FakeSignatureCssStyleProvider(
+        override val style: String = "<style>.mock-style { mock-property: mock-value }</style>",
+    ) : SignatureCssStyleProvider
 }
diff --git a/legacy/core/src/test/java/com/fsck/k9/message/html/HtmlConverterTest.kt b/legacy/core/src/test/java/com/fsck/k9/message/html/HtmlConverterTest.kt
index 50251127e83..78a60ebb199 100644
--- a/legacy/core/src/test/java/com/fsck/k9/message/html/HtmlConverterTest.kt
+++ b/legacy/core/src/test/java/com/fsck/k9/message/html/HtmlConverterTest.kt
@@ -2,45 +2,81 @@ package com.fsck.k9.message.html
 
 import assertk.assertThat
 import assertk.assertions.isEqualTo
+import com.fsck.k9.K9RobolectricTest
 import com.fsck.k9.mail.testing.crlf
 import com.fsck.k9.mail.testing.removeLineBreaks
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
+import org.intellij.lang.annotations.Language
+import org.junit.Before
+import org.junit.Rule
 import org.junit.Test
+import org.koin.test.mock.MockProviderRule
+import org.koin.test.mock.declareMock
+import org.mockito.Mockito
+import org.mockito.kotlin.doReturn
+import org.mockito.kotlin.mock
+import org.mockito.kotlin.whenever
+
+class HtmlConverterTest : K9RobolectricTest() {
+    private val mockedBlockquoteVarName = "--mocked-blockquote-var"
+    private val mockedPreClassName = "mocked-classname"
+
+    @get:Rule
+    val mockProvider = MockProviderRule.create { clazz ->
+        Mockito.mock(clazz.java)
+    }
+
+    @Before
+    fun setup() {
+        declareMock<CssVariableNameProvider> {
+            mock {
+                whenever(blockquoteDefaultBorderLeftColor) doReturn mockedBlockquoteVarName
+            }
+        }
+
+        declareMock<CssClassNameProvider> {
+            mock {
+                whenever(plainTextMessagePreClassName) doReturn mockedPreClassName
+            }
+        }
+    }
 
-class HtmlConverterTest {
     @Test
     fun `textToHtml() should convert quoted text using blockquote tags`() {
+        @Language("Markdown")
         val message =
             """
             Panama!
-            
+
             Bob Barker <bob@aol.com> wrote:
             > a canal
             >
             > Dorothy Jo Gideon <dorothy@aol.com> espoused:
             > >A man, a plan...
             > Too easy!
-            
+
             Nice job :)
             >> Guess!
             """.trimIndent().crlf()
 
         val result = HtmlConverter.textToHtml(message)
 
-        assertThat(result).isEqualTo(
-            """
-            |<pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            |<pre class="$mockedPreClassName">
             |<div dir="auto">
             |Panama!<br>
             |<br>
             |Bob Barker &lt;bob@aol.com&gt; wrote:<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #729fcf;">
             |<div dir="auto">
             | a canal<br>
             |<br>
             | Dorothy Jo Gideon &lt;dorothy@aol.com&gt; espoused:<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #ad7fa8;">
             |<div dir="auto">
             |A man, a plan...<br>
             |</div>
@@ -53,24 +89,25 @@ class HtmlConverterTest {
             |<br>
             |Nice job :)<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;">
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #729fcf;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #ad7fa8;">
             |<div dir="auto">
             |Guess!
             |</div>
             |</blockquote>
             |</blockquote>
             |</pre>
-            """.trimMargin().removeLineBreaks(),
-        )
+        """.trimMargin().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should retain indentation inside quoted text`() {
+        @Language("Markdown")
         val message =
             """
             *facepalm*
-            
+
             Bob Barker <bob@aol.com> wrote:
             > A wise man once said...
             >
@@ -81,15 +118,15 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(message)
 
-        assertThat(result).isEqualTo(
-            """
-            |<pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            |<pre class="$mockedPreClassName">
             |<div dir="auto">
             |*facepalm*<br>
             |<br>
             |Bob Barker &lt;bob@aol.com&gt; wrote:<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #729fcf;">
             |<div dir="auto">
             | A wise man once said...<br>
             |<br>
@@ -99,12 +136,13 @@ class HtmlConverterTest {
             |</div>
             |</blockquote>
             |</pre>
-            """.trimMargin().removeLineBreaks(),
-        )
+        """.trimMargin().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() with various quotation depths`() {
+        @Language("Markdown")
         val message =
             """
             zero
@@ -118,33 +156,33 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(message)
 
-        assertThat(result).isEqualTo(
-            """
-            |<pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            |<pre class="$mockedPreClassName">
             |<div dir="auto">
             |zero<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #729fcf;">
             |<div dir="auto">
             |one<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ad7fa8; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #ad7fa8;">
             |<div dir="auto">
             |two<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #8ae234; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #8ae234;">
             |<div dir="auto">
             |three<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #fcaf3e; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #fcaf3e;">
             |<div dir="auto">
             |four<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #e9b96e; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #e9b96e;">
             |<div dir="auto">
             |five<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #ccc; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #ccc;">
             |<div dir="auto">
             |six
             |</div>
@@ -155,12 +193,13 @@ class HtmlConverterTest {
             |</blockquote>
             |</blockquote>
             |</pre>
-            """.trimMargin().removeLineBreaks(),
-        )
+        """.trimMargin().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should preserve spaces at the start of a line`() {
+        @Language("Markdown")
         val message =
             """
             |foo
@@ -171,24 +210,25 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(message)
 
-        assertThat(result).isEqualTo(
-            """
-            |<pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            |<pre class="$mockedPreClassName">
             |<div dir="auto">
             |foo<br>
             | bar<br>
             |  baz<br>
             |</div>
             |</pre>
-            """.trimMargin().removeLineBreaks(),
-        )
+        """.trimMargin().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should preserve spaces at the start of a line followed by special characters`() {
+        @Language("Markdown")
         val message =
             """
-            | 
+            |${" "}
             |  &
             |   ${" "}
             |   <
@@ -198,27 +238,27 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(message)
 
-        assertThat(result).isEqualTo(
-            """
-            |<pre class="k9mail">
+        val expected = """
+            |<pre class="$mockedPreClassName">
             |<div dir="auto">
             | <br>
             |  &amp;<br>
             |    <br>
             |   &lt;<br>
             |</div>
-            |<blockquote class="gmail_quote" style="margin: 0pt 0pt 1ex 0.8ex; border-left: 1px solid #729fcf; padding-left: 1ex;">
+            |<blockquote class="gmail_quote" style="margin-bottom: 1ex; $mockedBlockquoteVarName: #729fcf;">
             |<div dir="auto">
             |<br>
             |</div>
             |</blockquote>
             |</pre>
-            """.trimMargin().removeLineBreaks(),
-        )
+        """.trimMargin().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should replace common horizontal divider ASCII patterns with HR tags`() {
+        @Language("Markdown")
         val text =
             """
             text
@@ -236,9 +276,9 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             text
             <hr>
@@ -253,12 +293,13 @@ class HtmlConverterTest {
             end
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should not convert dashes mixed with spaces to an HR tag`() {
+        @Language("Markdown")
         val text =
             """
             hello
@@ -268,21 +309,22 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             hello<br>
             --- --- --- --- ---<br>
             foo bar
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should merge consecutive horizontal dividers into a single HR tag`() {
+        @Language("Markdown")
         val text =
             """
             hello
@@ -293,35 +335,37 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             hello
             <hr>
             foo bar
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should not replace dashed horizontal divider prefixed with text`() {
+        @Language("Markdown")
         val text = "hello----\n\n"
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             hello----<br>
             <br>
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+        @Language("HTML")
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
@@ -330,7 +374,9 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo("""<pre class="k9mail"><div dir="auto">--<br></div></pre>""")
+        @Language("HTML")
+        val expected = """<pre class="$mockedPreClassName"><div dir="auto">--<br></div></pre>"""
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
@@ -339,7 +385,7 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo("""<pre class="k9mail"><div dir="auto">==<br></div></pre>""")
+        assertThat(result).isEqualTo("""<pre class="$mockedPreClassName"><div dir="auto">==<br></div></pre>""")
     }
 
     @Test
@@ -348,11 +394,14 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo("""<pre class="k9mail"><div dir="auto">__<br></div></pre>""")
+        @Language("HTML")
+        val expected = """<pre class="$mockedPreClassName"><div dir="auto">__<br></div></pre>"""
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should replace any combination of three consecutive divider characters with an HR tag`() {
+        @Language("Markdown")
         val text =
             """
             --=
@@ -364,47 +413,51 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo("""<pre class="k9mail"><div dir="auto"><hr></div></pre>""")
+        @Language("HTML")
+        val expected = """<pre class="$mockedPreClassName"><div dir="auto"><hr></div></pre>"""
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should replace dashes at the start of the input`() {
+        @Language("Markdown")
         val text = "---------------------------\nfoo bar"
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             <hr>
             foo bar
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should replace dashes at the end of the input`() {
+        @Language("Markdown")
         val text = "hello\n__________________________________"
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             hello
             <hr>
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should replace horizontal divider using ASCII scissors with an HR tag`() {
+        @Language("Markdown")
         val text =
             """
             hello
@@ -414,21 +467,22 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             hello
             <hr>
             world
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
     fun `textToHtml() should wrap email signature in a DIV`() {
+        @Language("Markdown")
         val text =
             """
             text
@@ -438,9 +492,9 @@ class HtmlConverterTest {
 
         val result = HtmlConverter.textToHtml(text)
 
-        assertThat(result).isEqualTo(
-            """
-            <pre class="k9mail">
+        @Language("HTML")
+        val expected = """
+            <pre class="$mockedPreClassName">
             <div dir="auto">
             text<br>
             <div class='k9mail-signature'>
@@ -449,8 +503,8 @@ class HtmlConverterTest {
             </div>
             </div>
             </pre>
-            """.trimIndent().removeLineBreaks(),
-        )
+        """.trimIndent().removeLineBreaks()
+        assertThat(result).isEqualTo(expected)
     }
 
     @Test
@@ -549,7 +603,7 @@ class HtmlConverterTest {
 
             Two
             Three
-            
+
             Four
             """.trimIndent(),
         )
diff --git a/legacy/ui/base/src/main/java/com/fsck/k9/ui/base/KoinModule.kt b/legacy/ui/base/src/main/java/com/fsck/k9/ui/base/KoinModule.kt
index 2878bb87847..bf5f3fff9cb 100644
--- a/legacy/ui/base/src/main/java/com/fsck/k9/ui/base/KoinModule.kt
+++ b/legacy/ui/base/src/main/java/com/fsck/k9/ui/base/KoinModule.kt
@@ -3,7 +3,9 @@ package com.fsck.k9.ui.base
 import com.fsck.k9.ui.base.locale.SystemLocaleManager
 import net.thunderbird.core.ui.theme.manager.ThemeManager
 import org.koin.core.qualifier.named
+import org.koin.dsl.bind
 import org.koin.dsl.module
+import net.thunderbird.core.ui.theme.api.ThemeManager as ThemeManagerApi
 
 val uiBaseModule = module {
     single {
@@ -13,7 +15,7 @@ val uiBaseModule = module {
             generalSettingsManager = get(),
             appCoroutineScope = get(named("AppCoroutineScope")),
         )
-    }
+    } bind ThemeManagerApi::class
     single { AppLanguageManager(systemLocaleManager = get(), displayCoreSettingsPreferenceManager = get()) }
     single { SystemLocaleManager(context = get()) }
 }
diff --git a/legacy/ui/legacy/build.gradle.kts b/legacy/ui/legacy/build.gradle.kts
index 52eccefa9ba..c76219c1aca 100644
--- a/legacy/ui/legacy/build.gradle.kts
+++ b/legacy/ui/legacy/build.gradle.kts
@@ -35,6 +35,7 @@ dependencies {
     implementation(projects.feature.mail.message.list)
     implementation(projects.feature.mail.message.composer)
     implementation(projects.feature.mail.message.export.api)
+    implementation(projects.feature.mail.message.reader.api)
 
     compileOnly(projects.mail.protocols.imap)
 
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/KoinModule.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/KoinModule.kt
index 3b533bbeae6..20ba8845d1c 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/KoinModule.kt
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/KoinModule.kt
@@ -3,5 +3,11 @@ package com.fsck.k9.activity
 import org.koin.dsl.module
 
 val activityModule = module {
-    single { MessageLoaderHelperFactory(messageViewInfoExtractorFactory = get(), htmlSettingsProvider = get()) }
+    single {
+        MessageLoaderHelperFactory(
+            messageViewInfoExtractorFactory = get(),
+            messageReaderHtmlSettingsProvider = get(),
+            messageComposerHtmlSettingsProvider = get(),
+        )
+    }
 }
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageLoaderHelperFactory.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageLoaderHelperFactory.kt
index 5e0caab781b..2e19b07ccb4 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageLoaderHelperFactory.kt
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageLoaderHelperFactory.kt
@@ -5,11 +5,13 @@ import androidx.fragment.app.FragmentManager
 import androidx.loader.app.LoaderManager
 import com.fsck.k9.activity.MessageLoaderHelper.MessageLoaderCallbacks
 import com.fsck.k9.mailstore.MessageViewInfoExtractorFactory
-import com.fsck.k9.ui.helper.HtmlSettingsProvider
+import net.thunderbird.feature.mail.message.composer.html.MessageComposerHtmlSettingsProvider
+import net.thunderbird.feature.mail.message.reader.api.html.MessageReaderHtmlSettingsProvider
 
 class MessageLoaderHelperFactory(
     private val messageViewInfoExtractorFactory: MessageViewInfoExtractorFactory,
-    private val htmlSettingsProvider: HtmlSettingsProvider,
+    private val messageReaderHtmlSettingsProvider: MessageReaderHtmlSettingsProvider,
+    private val messageComposerHtmlSettingsProvider: MessageComposerHtmlSettingsProvider,
 ) {
     fun createForMessageView(
         context: Context,
@@ -17,7 +19,7 @@ class MessageLoaderHelperFactory(
         fragmentManager: FragmentManager,
         callback: MessageLoaderCallbacks,
     ): MessageLoaderHelper {
-        val htmlSettings = htmlSettingsProvider.createForMessageView()
+        val htmlSettings = messageReaderHtmlSettingsProvider.create()
         val messageViewInfoExtractor = messageViewInfoExtractorFactory.create(htmlSettings)
         return MessageLoaderHelper(context, loaderManager, fragmentManager, callback, messageViewInfoExtractor)
     }
@@ -28,7 +30,7 @@ class MessageLoaderHelperFactory(
         fragmentManager: FragmentManager,
         callback: MessageLoaderCallbacks,
     ): MessageLoaderHelper {
-        val htmlSettings = htmlSettingsProvider.createForMessageCompose()
+        val htmlSettings = messageComposerHtmlSettingsProvider.create()
         val messageViewInfoExtractor = messageViewInfoExtractorFactory.create(htmlSettings)
         return MessageLoaderHelper(context, loaderManager, fragmentManager, callback, messageViewInfoExtractor)
     }
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/KoinModule.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/KoinModule.kt
index 84ce80f0133..113258af5ba 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/KoinModule.kt
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/KoinModule.kt
@@ -4,16 +4,22 @@ import android.content.Context
 import app.k9mail.legacy.message.controller.MessagingControllerMailChecker
 import com.fsck.k9.controller.MessagingController
 import com.fsck.k9.ui.helper.DisplayHtmlUiFactory
-import com.fsck.k9.ui.helper.HtmlSettingsProvider
 import com.fsck.k9.ui.helper.SizeFormatter
 import com.fsck.k9.ui.messageview.LinkTextHandler
 import com.fsck.k9.ui.share.ShareIntentBuilder
+import net.thunderbird.core.common.inject.getList
 import org.koin.core.qualifier.named
 import org.koin.dsl.module
 
 val uiModule = module {
-    single { HtmlSettingsProvider(themeManager = get(), generalSettingsManager = get()) }
-    single { DisplayHtmlUiFactory(get()) }
+    factory {
+        DisplayHtmlUiFactory(
+            cssClassNameProvider = get(),
+            cssStyleProviders = getList(),
+            messageReaderHtmlSettingsProvider = get(),
+            messageComposerHtmlSettingsProvider = get(),
+        )
+    }
     single<MessagingControllerMailChecker> { get<MessagingController>() }
     factory(named("MessageView")) { get<DisplayHtmlUiFactory>().createForMessageView() }
     factory { (context: Context) -> SizeFormatter(context.resources) }
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/DisplayHtmlUiFactory.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/DisplayHtmlUiFactory.kt
index 671981ee62f..91c0c60d917 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/DisplayHtmlUiFactory.kt
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/DisplayHtmlUiFactory.kt
@@ -1,13 +1,26 @@
 package com.fsck.k9.ui.helper
 
 import com.fsck.k9.message.html.DisplayHtml
+import net.thunderbird.feature.mail.message.composer.html.MessageComposerHtmlSettingsProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.html.MessageReaderHtmlSettingsProvider
 
-class DisplayHtmlUiFactory(private val htmlSettingsProvider: HtmlSettingsProvider) {
-    fun createForMessageView(): DisplayHtml {
-        return DisplayHtml(htmlSettingsProvider.createForMessageView())
-    }
+class DisplayHtmlUiFactory(
+    private val cssClassNameProvider: CssClassNameProvider,
+    private val cssStyleProviders: List<CssStyleProvider.Factory>,
+    private val messageReaderHtmlSettingsProvider: MessageReaderHtmlSettingsProvider,
+    private val messageComposerHtmlSettingsProvider: MessageComposerHtmlSettingsProvider,
+) {
+    fun createForMessageView(): DisplayHtml = DisplayHtml(
+        htmlSettings = messageReaderHtmlSettingsProvider.create(),
+        cssClassNameProvider = cssClassNameProvider,
+        cssStyleProviders = cssStyleProviders,
+    )
 
-    fun createForMessageCompose(): DisplayHtml {
-        return DisplayHtml(htmlSettingsProvider.createForMessageCompose())
-    }
+    fun createForMessageCompose(): DisplayHtml = DisplayHtml(
+        htmlSettings = messageComposerHtmlSettingsProvider.create(),
+        cssClassNameProvider = cssClassNameProvider,
+        cssStyleProviders = cssStyleProviders,
+    )
 }
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/HtmlSettingsProvider.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/HtmlSettingsProvider.kt
deleted file mode 100644
index 8596167c198..00000000000
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/ui/helper/HtmlSettingsProvider.kt
+++ /dev/null
@@ -1,21 +0,0 @@
-package com.fsck.k9.ui.helper
-
-import com.fsck.k9.message.html.HtmlSettings
-import net.thunderbird.core.preference.GeneralSettingsManager
-import net.thunderbird.core.ui.theme.api.Theme
-import net.thunderbird.core.ui.theme.manager.ThemeManager
-
-class HtmlSettingsProvider(
-    private val themeManager: ThemeManager,
-    private val generalSettingsManager: GeneralSettingsManager,
-) {
-    fun createForMessageView() = HtmlSettings(
-        useDarkMode = themeManager.messageViewTheme == Theme.DARK,
-        useFixedWidthFont = generalSettingsManager.getConfig().display.visualSettings.isUseMessageViewFixedWidthFont,
-    )
-
-    fun createForMessageCompose() = HtmlSettings(
-        useDarkMode = themeManager.messageComposeTheme == Theme.DARK,
-        useFixedWidthFont = false,
-    )
-}
diff --git a/legacy/ui/legacy/src/test/java/com/fsck/k9/TestApp.kt b/legacy/ui/legacy/src/test/java/com/fsck/k9/TestApp.kt
index e15fe16e6d6..bd0d76012bc 100644
--- a/legacy/ui/legacy/src/test/java/com/fsck/k9/TestApp.kt
+++ b/legacy/ui/legacy/src/test/java/com/fsck/k9/TestApp.kt
@@ -8,6 +8,7 @@ import net.thunderbird.core.android.account.AccountDefaultsProvider
 import net.thunderbird.core.android.account.LegacyAccountManager
 import net.thunderbird.core.android.preferences.TestStoragePersister
 import net.thunderbird.core.common.appConfig.PlatformConfigProvider
+import net.thunderbird.core.common.inject.factoryListOf
 import net.thunderbird.core.featureflag.FeatureFlag
 import net.thunderbird.core.featureflag.FeatureFlagProvider
 import net.thunderbird.core.featureflag.InMemoryFeatureFlagProvider
@@ -22,6 +23,9 @@ import net.thunderbird.core.logging.legacy.Log
 import net.thunderbird.core.logging.testing.TestLogLevelManager
 import net.thunderbird.core.logging.testing.TestLogger
 import net.thunderbird.core.preference.storage.StoragePersister
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssStyleProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
 import org.koin.core.qualifier.named
 import org.koin.dsl.bind
 import org.koin.dsl.module
@@ -83,6 +87,9 @@ val testModule = module {
     single<ContactPictureLoader> { mock() }
     single<LegacyAccountManager> { mock() }
     single<PlatformConfigProvider> { FakePlatformConfigProvider() }
+    single<CssVariableNameProvider> { mock() }
+    single<CssClassNameProvider> { mock() }
+    factoryListOf<CssStyleProvider>()
 }
 
 class FakePlatformConfigProvider : PlatformConfigProvider {
diff --git a/library/html-cleaner/build.gradle.kts b/library/html-cleaner/build.gradle.kts
index 976791cc02b..519d2d79578 100644
--- a/library/html-cleaner/build.gradle.kts
+++ b/library/html-cleaner/build.gradle.kts
@@ -5,4 +5,6 @@ plugins {
 
 dependencies {
     implementation(libs.jsoup)
+    implementation(projects.core.featureflag)
+    implementation(projects.feature.mail.message.reader.api)
 }
diff --git a/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt b/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt
index 2da379b4eca..665b12e52b0 100644
--- a/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt
+++ b/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt
@@ -1,13 +1,21 @@
 package app.k9mail.html.cleaner
 
+import net.thunderbird.core.featureflag.FeatureFlagProvider
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import org.jsoup.nodes.Document
 
-class HtmlProcessor(private val htmlHeadProvider: HtmlHeadProvider) {
+class HtmlProcessor(
+    private val featureFlagProvider: FeatureFlagProvider,
+    private val cssClassNameProvider: CssClassNameProvider,
+    private val htmlHeadProvider: HtmlHeadProvider,
+) {
     private val htmlSanitizer = HtmlSanitizer()
 
     fun processForDisplay(html: String): String {
         return htmlSanitizer.sanitize(html)
             .addCustomHeadContents()
+            .addCustomClasses()
             .toCompactString()
     }
 
@@ -22,4 +30,12 @@ class HtmlProcessor(private val htmlHeadProvider: HtmlHeadProvider) {
 
         return html()
     }
+
+    private fun Document.addCustomClasses() = apply {
+        if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageViewerCssStyles).isEnabled()) {
+            body()
+                .addClass(cssClassNameProvider.rootClassName)
+                .addClass(cssClassNameProvider.mainContentClassName)
+        }
+    }
 }

From d3e565fcac53c9b2cd673de2c69439ba35825085 Mon Sep 17 00:00:00 2001
From: Rafael Tonholo <rafael@tonholo.dev>
Date: Mon, 10 Nov 2025 08:47:02 -0400
Subject: [PATCH 8/8] chore(message-reader): remove html-cleaner feature
 project dependency

---
 cli/html-cleaner-cli/build.gradle.kts         |  2 --
 .../app/k9mail/cli/html/cleaner/Main.kt       | 25 +++++++------------
 .../k9/message/html/HtmlProcessorFactory.kt   |  9 ++++++-
 library/html-cleaner/build.gradle.kts         |  2 --
 .../app/k9mail/html/cleaner/HtmlProcessor.kt  | 14 ++++-------
 5 files changed, 22 insertions(+), 30 deletions(-)

diff --git a/cli/html-cleaner-cli/build.gradle.kts b/cli/html-cleaner-cli/build.gradle.kts
index 6f682f5adff..9f8b4e20d93 100644
--- a/cli/html-cleaner-cli/build.gradle.kts
+++ b/cli/html-cleaner-cli/build.gradle.kts
@@ -9,8 +9,6 @@ application {
 }
 
 dependencies {
-    implementation(projects.core.featureflag)
-    implementation(projects.feature.mail.message.reader.api)
     implementation(projects.library.htmlCleaner)
 
     implementation(libs.clikt)
diff --git a/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt b/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt
index 608ccbe3469..608ebb293a4 100644
--- a/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt
+++ b/cli/html-cleaner-cli/src/main/kotlin/app/k9mail/cli/html/cleaner/Main.kt
@@ -12,8 +12,6 @@ import com.github.ajalt.clikt.parameters.types.enum
 import com.github.ajalt.clikt.parameters.types.file
 import com.github.ajalt.clikt.parameters.types.inputStream
 import java.io.File
-import net.thunderbird.core.featureflag.FeatureFlagResult
-import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import okio.buffer
 import okio.sink
 import okio.source
@@ -45,21 +43,16 @@ class HtmlCleaner : CliktCommand() {
     }
 
     private fun cleanHtml(html: String): String {
+        val defaultNamespaceClassName = if (app == App.THUNDERBIRD) {
+            "net_thunderbird_android"
+        } else {
+            "com_fsck_k9"
+        }
         val htmlProcessor = HtmlProcessor(
-            featureFlagProvider = { FeatureFlagResult.Enabled },
-            cssClassNameProvider = object : CssClassNameProvider {
-                override val defaultNamespaceClassName: String
-                    get() = if (app == App.THUNDERBIRD) {
-                        "net_thunderbird_android"
-                    } else {
-                        "com_fsck_k9"
-                    }
-                override val rootClassName: String = "${defaultNamespaceClassName}__message-viewer"
-                override val mainContentClassName: String = "${defaultNamespaceClassName}__main-content"
-                override val plainTextMessagePreClassName: String =
-                    "${defaultNamespaceClassName}__plain-text-message-pre"
-                override val signatureClassName: String = "${defaultNamespaceClassName}__signature"
-            },
+            customClasses = setOf(
+                "${defaultNamespaceClassName}__message-viewer",
+                "${defaultNamespaceClassName}__main-content",
+            ),
             htmlHeadProvider = object : HtmlHeadProvider {
                 override val headHtml = """<meta name="viewport" content="width=device-width"/>"""
             },
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
index 06eaadfebd0..fdd03b2ab8e 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlProcessorFactory.kt
@@ -3,6 +3,7 @@ package com.fsck.k9.message.html
 import app.k9mail.html.cleaner.HtmlProcessor
 import net.thunderbird.core.common.mail.html.HtmlSettings
 import net.thunderbird.core.featureflag.FeatureFlagProvider
+import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
 import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 
 class HtmlProcessorFactory(
@@ -12,6 +13,12 @@ class HtmlProcessorFactory(
 ) {
     fun create(settings: HtmlSettings): HtmlProcessor {
         val displayHtml = displayHtmlFactory.create(settings)
-        return HtmlProcessor(featureFlagProvider, cssClassNameProvider, displayHtml)
+        val customClasses =
+            if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageReaderCssStyles).isEnabled()) {
+                setOf(cssClassNameProvider.rootClassName, cssClassNameProvider.mainContentClassName)
+            } else {
+                emptySet()
+            }
+        return HtmlProcessor(customClasses, displayHtml)
     }
 }
diff --git a/library/html-cleaner/build.gradle.kts b/library/html-cleaner/build.gradle.kts
index 519d2d79578..976791cc02b 100644
--- a/library/html-cleaner/build.gradle.kts
+++ b/library/html-cleaner/build.gradle.kts
@@ -5,6 +5,4 @@ plugins {
 
 dependencies {
     implementation(libs.jsoup)
-    implementation(projects.core.featureflag)
-    implementation(projects.feature.mail.message.reader.api)
 }
diff --git a/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt b/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt
index 665b12e52b0..daaf1d292bb 100644
--- a/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt
+++ b/library/html-cleaner/src/main/kotlin/app/k9mail/html/cleaner/HtmlProcessor.kt
@@ -1,13 +1,9 @@
 package app.k9mail.html.cleaner
 
-import net.thunderbird.core.featureflag.FeatureFlagProvider
-import net.thunderbird.feature.mail.message.reader.api.MessageReaderFeatureFlags
-import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
 import org.jsoup.nodes.Document
 
 class HtmlProcessor(
-    private val featureFlagProvider: FeatureFlagProvider,
-    private val cssClassNameProvider: CssClassNameProvider,
+    private val customClasses: Set<String>,
     private val htmlHeadProvider: HtmlHeadProvider,
 ) {
     private val htmlSanitizer = HtmlSanitizer()
@@ -32,10 +28,10 @@ class HtmlProcessor(
     }
 
     private fun Document.addCustomClasses() = apply {
-        if (featureFlagProvider.provide(MessageReaderFeatureFlags.UseNewMessageViewerCssStyles).isEnabled()) {
-            body()
-                .addClass(cssClassNameProvider.rootClassName)
-                .addClass(cssClassNameProvider.mainContentClassName)
+        if (customClasses.isNotEmpty()) {
+            body().apply {
+                customClasses.forEach(::addClass)
+            }
         }
     }
 }