Does not seem to work in Firefox 133.
I installed this for the first time in September and no overlay appears when I am in theatre mode on Twitch as described. I cannot see any settings or permissions that can be changed to get it working. Since this hasn't been updated for 2 1/2 years I guess it is broken.

This could be easily be a 5 Stars extension when transparency is working. The opacity slider does nothing, tried with & without BTTV, darkmode, lightmode.