@pastel10 ... Try this ..., in Settings, Enable Dark Theme. (which you have done).
Then under wallpapers, select a black wallpaper. That should work.
On my setup, the active tab is a dark gray background with white text.
The inactive tabs are black with white text. I also have minor vision problems and my setup makes it
much easier to read. Hope this helps.
One other thought, disable any tab extensions that might conflict with this.