This is a tiny Chrome extension that reverts GitHub.com's sans-serif font back to Helvetica. They changed it to "-apple-system" in July of 2016 and, if you're the kind of person who noticed, you might be the kind of person who wants to change it back.
This extension also includes a feature that sets tabs in GitHub's code view to equal whatever number of spaces you want (default: 4). GitHub defaults to 8 which is pretty silly, I think.
This is only meant for Mac devices.