Use Tauon

linux/nebula/configuration.nix

@@ -2,7 +2,7 @@
 # your system.  Help is available in the configuration.nix(5) man page
 # and in the NixOS manual (accessible by running `nixos-help`).
 
-{ ... }:
+{ pkgs, ... }:
 
 {
 
@@ -10,12 +10,7 @@
   boot.loader.grub = {
     enable = true;
     device = "nodev";
-    efiSupport = true;
-    efiInstallAsRemovable = true;
-    darkmatter-theme = {
-      enable = true;
-      style = "nixos";
-    };
+    theme = pkgs.sleek-grub-theme.override { withStyle = "dark"; };
   };
 
   # Set your time zone.

linux/nebula/default.nix

@@ -2,7 +2,6 @@
 
 {
   imports = with inputs; [
-    darkmatter.nixosModule
     disko.nixosModules.disko
     hardware.nixosModules.common-cpu-amd
     hardware.nixosModules.common-cpu-amd-pstate