Ralkage I thought about this more and how it could work within Flarum's options and UI. The Appearance tab assumes a single appearance is being used because Flarum isn't designed to have user selectable themes. Lots of these options become confusing when this toggle is introduced.
I made a mockup for how my ideas might be implemented in the admin panel through an extension. Admittedly this strays a bit from the name "Night Mode" but it covers the same and more use cases by addressing some of the potential problems I mentioned.
Sorry about the colors. I couldn't actually use these colors as-is with a toggle despite them looking decent in dark mode because that green looks terrible on a white background. As-is you need to pick your colors around this toggle when ideally you would be thinking about which colors work best together in each given appearance.