Because of the changes to FF 91 I've had to find something better than the default Dark theme and this is certainly it.
If I had any complaints I wish the background tabs could be a different color from the background itself, something in the dark grey, but I'm not sure if that's something you can do or just a fault of Firefox.
My real issue is that the Minimize, Maximize and Close buttons on some pages will disappear. If I switch tabs then it's fine but if I switch back they are gone again. I'm not sure if this is something on my end, maybe another extension causing it (I'm not sure how) or again if this is just a Firefox issue.