<Mist> I added the following code after importing the xmonad-contrib nix module:
imports = let inherit (inputs) xmonad-contrib;
in xmonad-contrib.nixosModules ++ [ xmonad-contrib.modernise.${system} ];
# ...
services.xserver.windowManager.xmonad = {
  enable = true;
  enableContribAndExtras = true;
  enableConfiguredRecompile = true;
  flake = {
    enable = true;
    compiler = "ghc947";
  };
};
And somehow I keep getting:
error: attribute 'enableContribAndExtras' missing