As a counterpoint, I find userstyles far less convenient than extensions. In my experience, they are more difficult to install, they work less often and break more easily (because of the "put this thing I hacked together on the internet and forget about it" mindset), don't auto update, are more difficult to find or discover, and the UI for managing/customizing them is horrible.

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.