[Slackwise]

Actually, if you have a Wikipedia account, you can save custom CSS (and even JS?!) to your account:

https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsec...

[Amorymeltzer]

Yes — custom gadgets and user scripts are quite common! It fits in with the ethos of the place, but is quite remarkable compared to nearly any other popular website.