[majorchord]

Isn't this just what userscripts/greasemonkey (and its forks) already does for any website?

[Izkata]

That's for javascript, your want Stylish for CSS:

https://addons.mozilla.org/en-US/firefox/addon/stylish/

https://chromewebstore.google.com/detail/stylish-custom-them...

[ranger_danger]

You can manipulate CSS/DOM with Javascript though.