To use this theme in NixOS set your SLiM configuration settings like so:
services.displayManager.slim = {
enable = true;
theme = pkgs.fetchurl {
url = "https://github.com/xzdandy/slim_theme_darkypink/archive/master.tar.gz";
sha256 = "c50f14cfed6a9fea090b613029ff43ac206e3107404071506260360596389eb7";
};
};