Comment by 65

You can also just use Tamper Monkey (though you need to add a script called Trusted Type Helper in order to inject CSS into YouTube).

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.