feat(grub): add theme

configuration.nix

@@ -4,7 +4,7 @@
   nixpkgs.config.allowUnfree = true;
 
   boot.loader = {
-    timeout = 1;
+    timeout = 5;
     efi.canTouchEfiVariables = true;
 
     grub = {
@@ -12,6 +12,10 @@
       device = "nodev";
       configurationLimit = 5;
       useOSProber = true;
+      theme = pkgs.sleek-grub-theme.override {
+        withBanner = "welcome to ${config.networking.hostName}";
+        withStyle = "dark";
+      };
     };
   };