gtk: add support for theming Flatpak apps

modules/gtk/hm.nix

@@ -1,4 +1,4 @@
-{ pkgs, config, lib, ... }:
+{ pkgs, config, lib, options, ... }:
 
 let
   cfg = config.stylix.targets.gtk;
@@ -12,7 +12,6 @@ let
     cat ${baseCss} >>$out
     echo ${lib.escapeShellArg cfg.extraCss} >>$out
   '';
-
 in {
   options.stylix.targets.gtk = {
     enable = config.lib.stylix.mkEnableTarget
@@ -29,25 +28,70 @@ in {
         window.background { border-radius: 0; }
       '';
     };
+
+    flatpakSupport.enable =
+      config.lib.stylix.mkEnableTarget "support for theming Flatpak apps" true;
   };
 
-  config = lib.mkIf cfg.enable {
-    # programs.dconf.enable = true; required in system config
-    gtk = {
-      enable = true;
-      font = {
-        inherit (config.stylix.fonts.sansSerif) package name;
-        size = config.stylix.fonts.sizes.applications;
+  config = lib.mkIf cfg.enable (lib.mkMerge [
+    {
+      # programs.dconf.enable = true; required in system config
+      gtk = {
+        enable = true;
+        font = {
+          inherit (config.stylix.fonts.sansSerif) package name;
+          size = config.stylix.fonts.sizes.applications;
+        };
+        theme = {
+          package = pkgs.adw-gtk3;
+          name = "adw-gtk3";
+        };
       };
-      theme = {
-        package = pkgs.adw-gtk3;
-        name = "adw-gtk3";
+
+      xdg.configFile = {
+        "gtk-3.0/gtk.css".source = finalCss;
+        "gtk-4.0/gtk.css".source = finalCss;
       };
-    };
+    }
 
-    xdg.configFile = {
-      "gtk-3.0/gtk.css".source = finalCss;
-      "gtk-4.0/gtk.css".source = finalCss;
-    };
-  };
+    (lib.mkIf cfg.flatpakSupport.enable (lib.mkMerge [
+      {
+        # Flatpak apps apparently don't consume the CSS config. This workaround appends it to the theme directly.
+        home.file.".themes/${config.gtk.theme.name}".source = pkgs.stdenvNoCC.mkDerivation {
+          name = "flattenedGtkTheme";
+          src = "${config.gtk.theme.package}/share/themes/${config.gtk.theme.name}";
+
+          installPhase = ''
+            cp --recursive . $out
+            cat ${finalCss} | tee --append $out/gtk-{3,4}.0/gtk.css
+          '';
+        };
+      }
+      (
+        let
+          filesystem = "${config.home.homeDirectory}/.themes/${config.gtk.theme.name}:ro";
+          theme = config.gtk.theme.name;
+        in
+          if options ? services.flatpak.overrides
+          then {
+            # Let Flatpak apps read the theme and force them to use it.
+            # This requires nix-flatpak to be imported externally.
+            services.flatpak.overrides.global = {
+              Context.filesystems = [filesystem];
+              Environment.GTK_THEME = theme;
+            };
+          }
+          else {
+            # This is likely incompatible with other modules that write to this file.
+            xdg.dataFile."flatpak/overrides/global".text = ''
+              [Context]
+              filesystems=${filesystem}
+
+              [Environment]
+              GTK_THEME=${theme}
+            '';
+          }
+      )
+    ]))
+  ]);
 }