I'm glad this exists, and very impressed with how front-and-center their source code is, but I decided a while ago that I'm not installing random add-ons unless I have a very high level of trust in their integrity.
Stylus is a godsend: https://github.com/openstyles/stylus. Sad thing is, this used to be a built-in browser feature.
Sure, there's advantages too, and I like the idea of user styles/scripts, but after wasting some time on trying (and failing) to use them to customize my browsing experience on a few sites, I settled on a Dark Reader + Auto Reader Mode + uBlock combo that makes all of the internet nice and readable with almost no extra hassle.