This is a very simple css modification for youtube music.
It is used with youtube music desktop app, it should work with every single OS and distro.
https://github.com/ytmdesktop/ytmdesktop
Just copy and paste the content of the theme.css onto the custom theme section on the youtube music desktop app.
You can find this by clicking the gear on the top right corner, then clicking appearance, and finally clicking the pencil on the custom theme option.
Remember to save and to activate custom theme option