So apparently uBlock Origin, with which I spent way too much time today, can be used to CHANGE a site's css.
And since I'm a bit paranoid about extensions, I decided to ditch Stylus and recreate @numimyon's colorful mastodon thing in UBO! And I added a rainbow TOOT! button!
Screenshot attached, link to uBlock Origin rules here: https://pastebin.com/raw/0KkvhshJ