diff --git a/modules/gtk/hm.nix b/modules/gtk/hm.nix
index 3a6fb22c..30facead 100644
--- a/modules/gtk/hm.nix
+++ b/modules/gtk/hm.nix
@@ -41,6 +41,10 @@ in
   config = lib.mkIf cfg.enable (
     lib.mkMerge [
       {
+        warnings =
+          lib.optional (config.gtk.gtk3.extraCss == "" && config.gtk.gtk4.extraCss == "")
+            "stylix: `gtk.gtk3.extraCss` and `gtk.gtk4.extraCss` have no effect. Use `stylix.targets.gtk.extraCss` instead.";
+
         # programs.dconf.enable = true; required in system config
         gtk = {
           enable = true;