pkgs:
with pkgs.lib;
let
  declToString = name: value: "  ${name}: ${toString value};";
  ruleToString = selector: decls:
                 "${selector} {\n" +
                 (concatStringsSep "\n" (mapAttrsToList declToString decls)) +
                 "\n}\n";
in
attrs: concatStrings (mapAttrsToList ruleToString attrs)