Modifying files or overwriting them?

Heya! I’m trying to figure out how to configure the cgit css file.

I have

  services.cgit.mccd = {
    settings = {
      css = "/cgit.css";

I’d like to overwrite this file with my own css file, and I am wondering what a good approach to do so is. Is there a way to import and copy over a file to the directory of the package that references it?