Repo: https://github.com/openstyles/stylus
Chrome extension: https://chrome.google.com/webstore/detail/stylus/clngdbkpkpe...
Firefox extension: https://addons.mozilla.org/firefox/addon/styl-us/
I used to use Stylebot but I switched everything over to Tamper Monkey so all my CSS and non CSS related scripts would be in one place.
Tamper Monkey/Grease Monkey scripts are very portable too, I use my scripts in Safari on iOS via the UserScripts extension.