A common misconception these days is that Flakes are somehow integral to everything. They’re just a more modular way of packaging nix code. What can be done with them can always be done without them.
Set boot.loader.grub.theme (you can find options like this yourself via search.nixos.org or with man configuration.nix).
There’s a decent chance something like a grub theme doesn’t even really have a build step and amounts to copying some files around. In that case it should be quite easy to package.
In fact, I went ahead and just did it. Add this to your configuration.nix and it should do the trick:
boot.loader.grub.theme = pkgs.stdenv.mkDerivation {
pname = "distro-grub-themes";
version = "3.1";
src = pkgs.fetchFromGitHub {
owner = "AdisonCavani";
repo = "distro-grub-themes";
rev = "v3.1";
hash = "sha256-ZcoGbbOMDDwjLhsvs77C7G7vINQnprdfI37a9ccrmPs=";
};
installPhase = "cp -r customize/nixos $out";
};