You can user userChrome.css [1][2][3]

You can start with this page[4] for an examples of simple, but elegant styling.

And /r/FirefoxCSS can demonstrate all kinds of crazy options userChome.css enthusiasts can come up with.

[1] https://www.userchrome.org/

[2] https://kb.mozillazine.org/index.php?title=UserChrome.css

[3] https://old.reddit.com/r/FirefoxCSS/wiki/index/tutorials

[4] https://www.userchrome.org/firefox-89-styling-proton-ui.html...