I don't know if github has an option for custom css but you can surely use a browser extension such as Stylus ([1], [2]) to change any website you want.

[1] for Firefox: https://addons.mozilla.org/en-US/firefox/addon/styl-us/

[2] for Chromium based browsers: https://chrome.google.com/webstore/detail/stylus/clngdbkpkpe...