gtk4: don't enable theme by default

modules/misc/gtk.nix

@@ -59,7 +59,7 @@ in
     theme = mkOption {
       type = types.nullOr themeType;
       default = null;
-      description = "Default theme for all GTK versions.";
+      description = "Default theme for GTK 2/3.";
     };
 
     iconTheme = mkOption {

modules/misc/gtk/gtk4.nix

@@ -45,9 +45,12 @@ in
           packageExample = "pkgs.gnome.gnome-themes-extra";
         }
       );
-      default = cfg.theme;
-      defaultText = literalExpression "config.gtk.theme";
-      description = "Theme for GTK 4 applications.";
+      default = null;
+      description = ''
+        Theme for GTK 4 applications.
+
+        Warning: This is not officially supported and applied using a workaround, and may cause issues.
+      '';
     };
 
     iconTheme = mkOption {

modules/misc/news/2025/09/2025-09-17_12-26-31.nix

@@ -0,0 +1,10 @@
+{
+  time = "2025-09-17T10:26:31+00:00";
+  condition = true;
+  message = ''
+    The option 'gtk.theme' does not apply to GTK 4 automatically anymore. Using a
+    custom theme is not officially supported by GTK 4, and is implemented by
+    home-manager using a workaround that may cause issues in some cases. If you
+    still want to use a GTK 4 theme, you need to explicitly set 'gtk.gtk4.theme'.
+  '';
+}

tests/modules/misc/gtk/gtk-global-inheritance-gtk4-expected.ini

@@ -5,4 +5,3 @@ gtk-cursor-theme-size=24
 gtk-font-name=Ubuntu 12
 gtk-icon-theme-name=Adwaita
 gtk-interface-color-scheme=2
-gtk-theme-name=Adwaita-dark

tests/modules/misc/gtk/gtk-global-inheritance.nix

@@ -37,9 +37,5 @@
     assertFileExists home-files/.config/gtk-4.0/settings.ini
     assertFileContent home-files/.config/gtk-4.0/settings.ini \
       ${./gtk-global-inheritance-gtk4-expected.ini}
-
-    # Check GTK4 CSS with theme import
-    assertFileExists home-files/.config/gtk-4.0/gtk.css
-    assertFileContent home-files/.config/gtk-4.0/gtk.css ${./gtk-global-inheritance-gtk4-css-expected.css}
   '';
 }

tests/modules/misc/gtk/gtk-per-version-override-gtk4-expected.ini

@@ -3,4 +3,3 @@ gtk-cursor-theme-name=Global-Cursor
 gtk-font-name=GTK4-Font 11
 gtk-icon-theme-name=Global-Icons
 gtk-recent-files-limit=20
-gtk-theme-name=Global-Theme

tests/modules/misc/gtk/gtk-per-version-override.nix

@@ -46,7 +46,7 @@
     assertFileContent home-files/.config/gtk-3.0/gtk.css \
       ${./gtk-per-version-override-gtk3-css-expected.css}
 
-    # GTK4 should use overridden font, global theme/icons/cursor
+    # GTK4 should use overridden font, global icons/cursor
     assertFileExists home-files/.config/gtk-4.0/settings.ini
     assertFileContent home-files/.config/gtk-4.0/settings.ini \
       ${./gtk-per-version-override-gtk4-expected.ini}

tests/modules/misc/gtk/gtk4/gtk4-basic-settings.nix

@@ -2,11 +2,11 @@
 {
   gtk = {
     enable = true;
-    theme = {
-      name = "catppuccin-macchiato-blue-standard";
-      package = pkgs.catppuccin-gtk;
-    };
     gtk4 = {
+      theme = {
+        name = "catppuccin-macchiato-blue-standard";
+        package = pkgs.catppuccin-gtk;
+      };
       extraConfig = {
         gtk-cursor-blink = false;
         gtk-recent-files-limit = 20;