I did this myself using userChrome.css :-)
Found the link for anyone else interested: https://www.userchrome.org/firefox-changes-userchrome-css.ht...
THANK YOU.
I had no idea this existed, I will be looking into this.
You have to enable it manually nowadays, but it opens up a ton of possibilities - you can theme virtually everything (find out which elements to theme using the browser toolbox).
Found the link for anyone else interested: https://www.userchrome.org/firefox-changes-userchrome-css.ht...