Dark mode themeing is always broken
Given that anything in HTML can specify the background colours and the viewer either respects them or throw away any colour information (e.g. keyword highlighting)
A good solution to blinding white screens is NegativeScreen from https://github.com/mlaily/NegativeScreen for Windows newer than XP. It even works with ClearType fonts. OK it's no good for photos but normal mode is just a win-alt-V away.