FINALLY!...an addon that works excellent in Ubuntu.
I was using Enhancer for YouTube, I made the bug report but no answer, so...if there's no interest from the developer, I uninstall the addon and give 1 star.
But this one works excellent!
Update: 1 star less, it has a bug, I have to reload/refresh YouTube for the theme to be applied.
I'm a Ubuntu 18.10 user.