Not plugins as in browser addons I don't think (tho those might exist too)

On mobile, no time for links, but you can google buuzzwords:

Checkout userscripts. A feature from a time when the experience of the web was still supposed to be hackable and not designed by corporate (you can in theory still create your own personal UI/UX for most sites). You'll want a userscript manager like tampermonkey. Disclaimer that if you don't understand javascript you could run malicious code, though the community provides some safety. If you want an extra safety layer copy/paste the code into ChatGPT and ask if its safe to install.

There is also a skin system I'll following up on