These are backed up, version-controlled override configurations for the Web Override Chrome extension.
Unfortunately, it seems that this extension is no longer available, which seems to happen somewhat often with these kinds of extensions. Its source code is still available, though, so it's possible to rebuild it manually if needed.
I still have it installed and am using it, however, I will probably be upgrading to something else at some point. In fact, I initially used Stylish for this purpose, but stopped using it after it became to spyware-ish for my taste. One extension I've seen mentioned in a few places is Resource Override, but even that seems to be in maintenance mode, with the last update released over a year ago.
My primary use-case for this is just making some stuff more readable on Gmail, so that's the only tweak currently available.
©2020 Milos Levacic