The easiest way to use these styles is to install one from userstyles.org, which supports most browsers. Find them here:
Currently diffs look pretty terrible. Any input on good colors for inserts and deletions is appreciated.
I'm not sure what some of the class names in GitHub's common.css denote, so in those cases I took a guess. I marked them with
??in the .scss source-- if you know what any of them are for, or can show me an example in the wild, I'd appreciate it.