Only wanted to change active tab colour in dark theme (actually "System theme, following operating system" which is set to dark mode).
Couldn't figure out what was meant by various items I can change.
It changed more than it indicated.
Neither Undo nor Revert actually work.
Opened issue in GitHub.
Disabled add-on until it can do this 1 simple and commonly wanted tweak.
Investigating userchrome.css