gnome: accent color is no longer themed

[danth]

Elements which use the accent color are now always the default blue.

This happens because GNOME 47 added support for a configurable accent color. The CSS now contains placeholders, namely -st-accent-color and -st-accent-fg-color, which are substituted with the configured accent color at runtime.

• We can't just change the accent color through Dconf, as it uses a hardcoded list of choices.
• We can't just replace the placeholders in the CSS with a hardcoded value, as this JavaScript code also makes use of the placeholders.

To progress, we either need to:

• Replace the choices for the accent color with colors from our theme
• Patch the JavaScript code to not use -st-accent-color

Both of these require patching GNOME Shell, which is not possible with Home Manager alone due to it being installed at the operating system level.

If we remove Home Manager support, we lose:

• The ability to theme GNOME at all on non-NixOS
• The ability to have per-user themes on NixOS

Reference: https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2715

[danth]

For reference, this hardcodes a value into the CSS, but would require patching the JavaScript (and possibly other unknowns) to be fully effective.

diff --git a/modules/gnome/theme.nix b/modules/gnome/theme.nix
index 6a6ea7c..41c61d3 100644
--- a/modules/gnome/theme.nix
+++ b/modules/gnome/theme.nix
@@ -13,6 +13,15 @@ in stdenv.mkDerivation {
   postPatch = ''
     rm data/theme/gnome-shell-sass/{_colors.scss,_default-colors.scss,_palette.scss}
     cp ${colorsScss} data/theme/gnome-shell-sass/_colors.scss
+
+    substituteInPlace \
+      data/theme/gnome-shell-sass/**/*.scss \
+      --replace-quiet '-st-accent-color' '#${colors.base0D-hex}' \
+      --replace-quiet '-st-accent-fg-color' '#${colors.base00-hex}' \
+      --replace-quiet 'st-transparentize' 'transparentize' \
+      --replace-quiet 'st-mix' 'mix' \
+      --replace-quiet 'st-darken' 'darken' \
+      --replace-quiet 'st-lighten' 'lighten'
   '';
 
   nativeBuildInputs = [ sass glib.dev ];