fix(nixos/sddm): use the kde sddm package

modules/nixos/sddm.nix

@@ -40,10 +40,17 @@ in
       default = true;
       description = "Add an additional background layer to the login panel";
     };
+
+    assertQt6Sddm =
+      lib.mkEnableOption "checking `services.displayManager.sddm.package` is the Qt 6 version. This is to ensure the theme is applied properly, but may have false positives in the case of overridden packages for example"
+      // {
+        default = true;
+      };
   };
 
   config = mkIf enable {
     services.displayManager.sddm.theme = "catppuccin-${cfg.flavor}";
+
     environment.systemPackages = [
       (pkgs.catppuccin-sddm.override {
         inherit (cfg)
@@ -55,5 +62,12 @@ in
           ;
       })
     ];
+
+    assertions = lib.lists.optionals cfg.assertQt6Sddm [
+      {
+        assertion = config.services.displayManager.sddm.package != pkgs.kdePackages.sddm;
+        message = "Not using the kdePackages.sddm package, will cause the theme to fail being applied";
+      }
+    ];
   };
 }