feat(themes): declaratively manage icons, cursors, and dark/light modes

- Automatically detect light mode via 'light.mode' file in theme folder
- Read icon theme name from 'icons.theme' file in theme folder
- Switch Stylix polarity and iconTheme based on theme selection
- Add yaru-theme and bibata-cursors to default packages
- Remove obsolete imperative nomarchy-theme-set-gnome script

modules/home/options.nix

@@ -1,4 +1,4 @@
-{ lib, ... }:
+{ lib, pkgs, ... }:
 
 {
   options.nomarchy = {
@@ -73,6 +73,28 @@
         description = "System monospace font.";
       };
     };
+    iconsTheme = lib.mkOption {
+      type = lib.types.str;
+      default = "Yaru-blue";
+      description = "System icon theme name.";
+    };
+    isLightMode = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = "Whether the current theme is light mode.";
+    };
+    cursor = {
+      name = lib.mkOption {
+        type = lib.types.str;
+        default = "Bibata-Modern-Ice";
+        description = "Mouse cursor theme name.";
+      };
+      package = lib.mkOption {
+        type = lib.types.package;
+        default = pkgs.bibata-cursors;
+        description = "Package providing the cursor theme.";
+      };
+    };
     configOverrides = lib.mkOption {
       type = lib.types.nullOr lib.types.path;
       default = null;