This is a flake of Grub themes to use with NixOS. Each of the directories in themes gives a theme which is usable.
It is simple to add themes to this flake! Just add a directory to themes and put a default.nix file in there. Feel free to submit a pull request to add themes.
To use one, set the boot.loader.grub.theme option in your NixOS configuration to the desired derivation generated by this flake.