I've just downloaded a dev VM, set the dark theme, but I see the tabs in light (sorta known bug, that might be solved when fixing the scrollbar problem):
Tried changing to light mode, no difference. Still no title bars.
What is also interesting is that new tabs do not show anything.
If you look I have the settings tab open, the tab to the left is the browser homepage but it is totally black.
Should say "Mullvad Browser Home" I expect.