I'm not saying occasionally at night when it's darker. A bright screen in an otherwise dark environment is not comfortable, so a dark theme makes sense. Similarly, working in daylight conditions with a dark theme is not comfortable either, it puts an unnecessary strain on the eyes that need more light to discern the details but can't open more because of the ambient light.For me working with dark backgrounds is much easier on the eyes than light, especially if you're used to working into the night. Hasn't been in any way detrimental for my vision, which has been more or less consistent for years, so I think that that's a myth that your vision will deteriorate if you use a dark background.
As for reading or watching a screen in dark conditions, there are enough studies showing that it strains the eyes, causes more frequent blinking and more fatigue. I don't know if that's detrimental in the long term but there is certainly no upside vs good lighting conditions.
The relative brightness of text vs background in the theme is important too, that's what we discussed at the beginning of this thread. A text that is too bright causes discomfort because it's harder for the eye to focus on bright dots and lines, and so it looks fuzzier. The new dark theme is much better than the old one in that regard, no wonder the number of fans has increased so much.