{
  description = "Firefox userChrome.css";
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  outputs =
    { self, nixpkgs }:
    {
      userChrome = builtins.readFile ./userChrome.css;

      # `nix fmt` formats userChrome.css with prettier
      formatter = nixpkgs.lib.mapAttrs (
        system: pkgs:
        pkgs.writeShellApplication {
          name = "fmt";
          runtimeInputs = [ pkgs.prettier ];
          text = ''
            prettier --write userChrome.css
          '';
        }
      ) nixpkgs.legacyPackages;
    };
}