home/catppuccin+nixos/nerdfonts: enable gtk-related stuff

modules/programs/catppuccin.nix

@@ -48,5 +48,10 @@
       enable = true;
       style.name = "kvantum";
     };
+
+    gtk = {
+      enable = true;
+      colorScheme = "dark";
+    };
   };
 }