some minor refactor

modules/options/theme/default.nix

@@ -0,0 +1,28 @@
+{
+  lib,
+  config,
+  ...
+}: let
+  inherit (lib) mkOption types;
+  bgs = {
+    wallpaper_1 = "~/media/images/bg_1.jpg";
+    wallpaper_2 = "~/media/images/bg_2.jpg";
+    wallpaper_3 = "~/media/images/bg_3.jpg";
+  };
+  bgList = builtins.attrNames bgs;
+in {
+  options.theme = {
+    background = {
+      lockscreen = mkOption {
+        type = types.enum bgList;
+        apply = name: bgs.${name};
+        example = "wallpaper_2";
+      };
+      desktop = mkOption {
+        type = types.enum bgList;
+        apply = name: bgs.${name};
+        example = "wallpaper_1";
+      };
+    };
+  };
+}