This is a tiny Chrome extension that reverts GitHub.com's sans-serif font back to Helvetica. It was changed to to "-webkit-default" 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 (defaulting to four). GitHub defaults to eight spaces per tab which is pretty silly, I think.
Option 1: Just install from the Google Web Store!
Option 2 (more advanced): You can also install the latest, unpacked version directly from this GitHub repo. Clone or download-and-unzip this repository, then click "load unpacked extension" in Google Chrome, and select the github-style-override-chrome directory you just downloaded. That's it! See https://developer.chrome.com/extensions/getstarted#unpacked for step-by-step instructions on installing an unpacked extension, if you need them.
- This is meant for Chrome running on Mac. Windows probably won't work (and won't display Helvetica anyway unless you went out and licensed it).
- I'll keep this updated indefinitely. GitHub on Mac+Chrome comprises a large part of my daily career activities and I can't look at San Francisco all day.
- Feel free to contribute. If you're the kind of person who noticed this stuff, you're probably the kind of person who will have good stuff to add.