added flake.nix and README instruction for nixos

README.md

@@ -53,6 +53,42 @@ For Debian, you can follow [this link](https://vitux.com/install-grub-customizer
 
 Install grub themes with `grub-customizer` tool
 
+## NixOS installation
+
+- Add this flake input to your **flake.nix** inputs
+```nix
+inputs = {
+    grub-theme = {
+      url = "github:xenlism/Grub-themes";
+    };
+
+    # More inputs ...
+};
+```
+
+> [!IMPORTANT]
+> If you switched your theme from systemd-boot to GRUB.
+> You have to change your boot entry in UEFI or BIOS to GRUB.
+> Because NixOS will otherwise still boot into systemd-boot.
+
+- Then set GRUB theme to this package in your **configuration.nix** file
+```nix
+boot.loader.grub.theme = inputs.grub-theme.packages.x86_64-linux.xenlism-grub-theme-nixos-1080p;
+```
+
+> [!NOTE]
+> Just a side note, in case you are using stylix or something that sets GRUB theme for you.
+> You will maybe have to set this option with *lib.mkForce* to make it override other value.
+
+- You can pick from list of distros and resolutions that are present in the [original repo](https://github.com/xenlism/Grub-themes/)
+
+- To view all possible options run `nix flake show github:xenlism/Grub-themes --no-write-lock-file` this will display all possible packages to install
+
+- To change theme you just have to change the package name in **configuration.nix** option like this:
+```nix
+boot.loader.grub.theme = inputs.grub-theme.packages.x86_64-linux.xenlism-grub-theme-[distribution]-[resolution];
+```
+
 ## Preview
 
 | Ubuntu                                                       | Mint Linux                                                   |

flake.nix

@@ -0,0 +1,73 @@
+{
+  description = "Xenlism GRUB themes — packaged for NixOS";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs }: 
+    let
+      system = "x86_64-linux";
+      pkgs = import nixpkgs { inherit system; };
+      lib = pkgs.lib;
+
+
+      # supported resolutions and distros
+      resolutions = [ "1080p" "2k" "4k" ];
+      distros = [
+        "arch" "Debian" "Fedora" "gentoo" "kali" "mac" "Manjaro"
+        "mint" "nixos" "opensuse" "popos" "ubuntu" "win10" "win11"
+      ];
+
+      mkTheme = { distro, resolution }: pkgs.stdenvNoCC.mkDerivation {
+        pname = "xenlism-grub-theme-${distro}-${resolution}";
+        version = "2025-10-18";
+
+        src = pkgs.fetchFromGitHub {
+          owner = "xenlism";
+          repo = "Grub-themes";
+          rev = "40ac048df9aacfc053c515b97fcd24af1a06762f";
+          hash = "sha256-ProTKsFocIxWAFbYgQ46A+GVZ7mUHXxZrvdiPJqZJ6I=";
+        };
+
+        dontBuild = true;
+
+        installPhase = ''
+          runHook preInstall
+
+          # Construct path like xenlism-grub-1080p-nixos/
+          srcDir="xenlism-grub-${resolution}-${distro}"
+          themeDir="$out"
+
+          mkdir -p "$themeDir"
+          echo "Copying theme from $srcDir"
+          cp -r "$srcDir"/Xenlism-${lib.toUpper (lib.substring 0 1 distro)}${lib.substring 1 (lib.stringLength distro - 1) distro}/* "$themeDir/"
+
+          runHook postInstall
+        '';
+
+        meta = {
+          description = "Xenlism GRUB theme for ${lib.toUpper distro} (${resolution})";
+          homepage = "https://github.com/xenlism/Grub-themes";
+          #license = lib.licenses.gpl3Plus;
+          platforms = lib.platforms.linux;
+          maintainers = [];
+        };
+      };
+
+      allThemes = builtins.listToAttrs (
+        builtins.concatLists (map (distro:
+          map (resolution: {
+            name = "xenlism-grub-theme-${distro}-${resolution}";
+            value = mkTheme { inherit distro resolution; };
+          }) resolutions
+        ) distros)
+      );
+
+    in {
+      packages.${system} = allThemes // {
+        default = allThemes."xenlism-grub-theme-nixos-1080p";
+      };
+    };
+}
+