diff --git a/index.html b/index.html
index afbee53..2d55189 100644
--- a/index.html
+++ b/index.html
@@ -7,6 +7,7 @@
     <meta http-equiv="content-style-type" content="text/css">
     <meta name="description" content="This is the online markdown editor with live preview.">
 
+    <link id="markdown-theme-css" rel="stylesheet" href="https://cdn.jsdelivr.net/npm/github-markdown-css@5.8.1/github-markdown-light.min.css">
     <link rel="stylesheet" type="text/css" href="css/style.css?v=1.10.1">
     <link rel="icon" type="image/png" href="favicon.png">
     <script type="module" src="/src/main.js?v=1.10.1"></script>
@@ -31,6 +32,7 @@
             <div id="reset-button"><a href="#">Reset</a></div>
             <div id="copy-button"><a href="#">Copy</a></div>
             <div id="sync-button"><input type="checkbox" id="sync-scroll-checkbox"><label for="sync-scroll-checkbox">Sync scroll</label></div>
+            <div id="theme-button"><input type="checkbox" id="theme-checkbox"><label for="theme-checkbox">Dark mode</label></div>
         </div>
         <div id="github"><a href="https://github.com/tanabe/markdown-live-preview"><img src="image/GitHub-Mark-Light-32px.webp"></a></div>
     </header>
diff --git a/public/css/style.css b/public/css/style.css
index 6187a31..ac42b23 100644
--- a/public/css/style.css
+++ b/public/css/style.css
@@ -1,12 +1,112 @@
+:root {
+  --bg-color: #fafafa;
+  --text-color: #24292e;
+  --header-bg: #3a3a3a;
+  --header-text: #f6f8fa;
+  --border-color: #e1e4e8;
+  --divider-color: #d1d5da;
+  --divider-hover: #959da5;
+  --divider-active: #6a737d;
+}
+
+[data-theme="dark"] {
+  --bg-color: #212121;
+  --text-color: #E8E8E8;
+  --header-bg: #1a1a1a;
+  --header-text: #E8E8E8;
+  --border-color: #555555;
+  --divider-color: #4a4a4a;
+  --divider-hover: #5a5a5a;
+  --divider-active: #6a6a6a;
+}
+
+/* Dark mode overrides for GitHub markdown CSS */
+[data-theme="dark"] .markdown-body {
+  color: #E8E8E8 !important;
+  background-color: #212121 !important;
+}
+
+[data-theme="dark"] #output {
+  background-color: #212121 !important;
+  color: #E8E8E8 !important;
+}
+
+/* Headings */
+[data-theme="dark"] .markdown-body h1,
+[data-theme="dark"] .markdown-body h2,
+[data-theme="dark"] .markdown-body h3,
+[data-theme="dark"] .markdown-body h4,
+[data-theme="dark"] .markdown-body h5,
+[data-theme="dark"] .markdown-body h6 {
+  color: #F5F5F5 !important;
+  border-color: #555555 !important;
+}
+
+/* Blockquotes */
+[data-theme="dark"] .markdown-body blockquote {
+  color: #B8B8B8 !important;
+  border-left-color: #555555 !important;
+}
+
+/* Code */
+[data-theme="dark"] .markdown-body code {
+  background-color: rgba(255, 255, 255, 0.1) !important;
+  color: #F5F5F5 !important;
+}
+
+[data-theme="dark"] .markdown-body pre {
+  background-color: #1a1a1a !important;
+  border-color: #555555 !important;
+}
+
+[data-theme="dark"] .markdown-body pre code {
+  background-color: transparent !important;
+}
+
+/* Tables */
+[data-theme="dark"] .markdown-body table {
+  color: #E8E8E8 !important;
+}
+
+[data-theme="dark"] .markdown-body table th,
+[data-theme="dark"] .markdown-body table td {
+  border-color: #555555 !important;
+}
+
+[data-theme="dark"] .markdown-body table tr {
+  background-color: transparent !important;
+  border-color: #555555 !important;
+}
+
+[data-theme="dark"] .markdown-body table tr:nth-child(2n) {
+  background-color: rgba(255, 255, 255, 0.03) !important;
+}
+
+[data-theme="dark"] .markdown-body table th {
+  background-color: rgba(255, 255, 255, 0.08) !important;
+}
+
+/* Links */
+[data-theme="dark"] .markdown-body a {
+  color: #58a6ff !important;
+}
+
+/* Horizontal rules */
+[data-theme="dark"] .markdown-body hr {
+  background-color: #555555 !important;
+  border-color: #555555 !important;
+}
+
+
 html, body {
-  color: #333;
+  color: var(--text-color);
   overflow-x: hidden;
   margin: 0;
   padding: 0;
   font-family: Helvetica, arial, freesans, clean, sans-serif;
   font-size: 1em;
   line-height: 1.5em;
-  background-color: #fff;
+  background-color: var(--bg-color);
   overscroll-behavior-y: none;
 }
 
@@ -31,7 +131,7 @@ a:link,
 a:visited,
 a:hover
 a:active {
-  color: #333;
+  color: var(--text-color);
   text-decoration: none;
 }
 
@@ -45,13 +145,13 @@ header {
   align-items: center;
   padding: 8px 16px;
   width: 100%;
-  background-color: #444;
+  background-color: var(--header-bg);
   font-size: 12px;
-  color: #fff;
+  color: var(--header-text);
 }
 
 header a:link, header a:hover, header a:visited, header a:active {
-  color: #fff;
+  color: var(--header-text);
 }
 
 header a:hover {
@@ -78,6 +178,14 @@ header a:hover {
     user-select: none;
 }
 
+#theme-button {
+    margin-left: 16px;
+    -webkit-user-select: none;
+    -moz-user-select: none;
+    -ms-user-select: none;
+    user-select: none;
+}
+
 header input[type="checkbox"] {
     vertical-align: middle;
     margin-right: 4px;
@@ -103,8 +211,8 @@ footer {
   bottom: 0;
   left: 0;
   width: 100%;
-  background-color: #fff;
-  color: #fff;
+  background-color: var(--bg-color);
+  color: var(--header-text);
   text-align: center;
   justify-content: center;
   align-items: center;
@@ -115,8 +223,8 @@ footer {
   overflow: hidden;
   width: 100%;
   margin: 0 auto;
-  background-color: #fff;
-  border-bottom: 1px solid #e8e8e8;
+  background-color: var(--bg-color);
+  border-bottom: 1px solid var(--border-color);
   display: flex;
 }
 
@@ -130,6 +238,8 @@ footer {
 
 #preview-wrapper {
   padding: 8px 16px 16px 16px;
+  background-color: var(--bg-color);
+  color: var(--text-color);
 }
 
 .column {
@@ -143,6 +253,7 @@ footer {
 #preview.column {
   overflow-y: scroll;
   white-space: normal;
+  background-color: var(--bg-color);
 }
 
 .split-container {
@@ -158,15 +269,15 @@ footer {
 
 .split-divider {
   width: 5px;
-  background: #ccc;
+  background: var(--divider-color);
   cursor: col-resize;
   z-index: 1;
 }
 
 .split-divider.hover {
-  background: #999;
+  background: var(--divider-hover);
 }
 
 .split-divider.active {
-  background: #666;
-}
\ No newline at end of file
+  background: var(--divider-active);
+}
diff --git a/src/main.js b/src/main.js
index 2aa4626..4a78782 100644
--- a/src/main.js
+++ b/src/main.js
@@ -2,7 +2,6 @@ import Storehouse from 'storehouse-js';
 import * as monaco from 'https://cdn.jsdelivr.net/npm/monaco-editor@0.52.2/+esm';
 import { marked } from 'marked';
 import DOMPurify from 'dompurify';
-import 'github-markdown-css/github-markdown-light.css';
 
 const init = () => {
     let hasEdited = false;
@@ -11,6 +10,7 @@ const init = () => {
     const localStorageNamespace = 'com.markdownlivepreview';
     const localStorageKey = 'last_state';
     const localStorageScrollBarKey = 'scroll_bar_settings';
+    const localStorageThemeKey = 'theme_preference';
     const confirmationMessage = 'Are you sure you want to reset? Your changes will be lost.';
     // default template
     const defaultInput = `# Markdown syntax guide
@@ -105,7 +105,8 @@ This web site is using ${"`"}markedjs/marked${"`"}.
             hover: { enabled: false },
             quickSuggestions: false,
             suggestOnTriggerCharacters: false,
-            folding: false
+            folding: false,
+            theme: 'vs'
         });
 
         editor.onDidChangeModelContent(() => {
@@ -260,6 +261,49 @@ This web site is using ${"`"}markedjs/marked${"`"}.
         Storehouse.setItem(localStorageNamespace, localStorageScrollBarKey, settings, expiredAt);
     };
 
+    let loadThemePreference = () => {
+        let theme = Storehouse.getItem(localStorageNamespace, localStorageThemeKey);
+        return theme || 'light';
+    };
+
+    let saveThemePreference = (theme) => {
+        let expiredAt = new Date(2099, 1, 1);
+        Storehouse.setItem(localStorageNamespace, localStorageThemeKey, theme, expiredAt);
+    };
+
+    let applyTheme = (theme) => {
+        // Update data attribute for CSS variables
+        document.documentElement.setAttribute('data-theme', theme === 'dark' ? 'dark' : '');
+        
+        // Update Monaco editor theme
+        monaco.editor.setTheme(theme === 'dark' ? 'vs-dark' : 'vs');
+        
+        // Update markdown CSS link
+        const markdownLink = document.getElementById('markdown-theme-css');
+        if (markdownLink) {
+            if (theme === 'dark') {
+                markdownLink.href = 'https://cdn.jsdelivr.net/npm/github-markdown-css@5.8.1/github-markdown-dark.min.css';
+            } else {
+                markdownLink.href = 'https://cdn.jsdelivr.net/npm/github-markdown-css@5.8.1/github-markdown-light.min.css';
+            }
+        }
+    };
+
+    let setupThemeToggle = () => {
+        const themeCheckbox = document.getElementById('theme-checkbox');
+        const currentTheme = loadThemePreference();
+        
+        // Set initial checkbox state
+        themeCheckbox.checked = currentTheme === 'dark';
+        
+        // Handle theme toggle
+        themeCheckbox.addEventListener('change', (event) => {
+            const newTheme = event.target.checked ? 'dark' : 'light';
+            saveThemePreference(newTheme);
+            applyTheme(newTheme);
+        });
+    };
+
     let setupDivider = () => {
         let lastLeftRatio = 0.5;
         const divider = document.getElementById('split-divider');
@@ -346,6 +390,14 @@ This web site is using ${"`"}markedjs/marked${"`"}.
     }
     setupResetButton();
     setupCopyButton(editor);
+    
+    // Initialize theme
+    const currentTheme = loadThemePreference();
+    
+    // Apply theme settings
+    applyTheme(currentTheme);
+    
+    setupThemeToggle();
 
     let scrollBarSettings = loadScrollBarSettings() || false;
     initScrollBarSync(scrollBarSettings);