@zens Keep in mind that the two browsers with greatest use today, Safari/iOS and Chrome/Android do not support extensions at all.
Firefox/Android (or Fennec/Android) do support LIMITED extensions. This includes DarkReader (previously mentioned), but not either Stylus (CSS manager) or Tampermonkey (Javascript manager).
My strongly preferred e-ink browser, EinkBro, does not support extensions at all, though it has some integrated tools: font zoom, White Background, JS toggle, and a few others.
(It also defaults to having a touch-area based page-based navigation, which is the real killer feature, as scroll and e-ink play very, very, very poorly. Yes, you can still scroll, but it's not necessary much of the time, and there are other options.)
So no, Tampermonkey or similar CSS/JS managers really doesn't address the problem here. I'd love it if they did.
@dredmorbius do bookmarklets work?