It's working for me in macOS and Windows using the latest Firefox Beta, using either theme "Dark" or theme "Light".
If you have set Firefox to use theme "System auto - theme", it won't work - as clearly stated in the "About this extension" section.

> Keeps consistent across theme changes and works with default Firefox light and dark themes (doesn't work with the most recent Firefox Alpenglow theme or if "System auto - theme" is set).

There are limitations presented when using "System auto - theme" which doesn't allow a straightforward customization.

I will update description to clearly state at the top this info. Please review your rating if that is the case.