regreet: respect dark mode and do not set extraCss

ReGreet will choose dark colors from the GTK theme if we instruct it to
do so.
ReGreet uses the global GTK theme if the environment is set the default
Nixpkgs configuration takes care of that so extraCss will be left free
for actual user overrides.

modules/regreet/nixos.nix

@@ -12,7 +12,22 @@
     lib.mkIf
       (config.stylix.enable && config.stylix.targets.regreet.enable && pkgs.stdenv.hostPlatform.isLinux)
       {
+        warnings =
+          let
+            cfg = config.programs.regreet;
+          in
+          lib.mkIf
+            (
+              cfg.enable
+              &&
+                !config.services.greetd.settings.default_session.command
+                == "${pkgs.dbus}/bin/dbus-run-session ${lib.getExe pkgs.cage} ${lib.escapeShellArgs cfg.cageArgs} -- ${lib.getExe cfg.package}"
+            )
+            [
+              "Stylix is not guaranteed to style regreet correctly when setting a custom command in `services.greetd.settings.default_session.command `. Note that in most cases no variables under `services.greetd` need to be manually set to ensure that ReGreet is functional"
+            ];
         programs.regreet = {
+          settings.GTK.application_prefer_dark_theme = config.stylix.polarity == "dark";
           settings.background = {
             path = config.stylix.image;
             fit = let
@@ -37,10 +52,6 @@
             package = pkgs.adw-gtk3;
             name = "adw-gtk3";
           };
-          extraCss = config.lib.stylix.colors {
-            template = ./../gtk/gtk.mustache;
-            extension = "css";
-          };
         };
       };
 }