You say you can't fathom it, but then you seem to subsequently fathom it. Dark themes are nice in dark rooms. If you assume most people spend most of their screen time in well lit environments, then puzzlement might seem warranted. But why assume something which leads to perplexing conclusions? The obvious answer is that people who prefer dark themes also tend to work in dark rooms.

When I'm on my balcony in the sun, I use a light theme. But that's only a few hours a day during the summer. Most of the time I'm indoors with the lights low and my themes dark.

I mostly see people at work using dark mode during the day time and cheering anytime anything gets dark mode.

I get it in dark rooms, but I get the sense there is this trend where people like dark mode because they like dark mode and they don't care what time of day it is. Seems more like fashion than anything practical, though I can certainly understand there is a practical application to it, and giving users control of it is good.

Indoors 'during the daytime' is still pretty dark in a great many rooms, often due to the preference of the rooms occupants. So it really has little to do with the time of day. Right now it's early in the afternoon for me; the sun is still up but it's overcast, my window blinds closed, and my lights aren't turned on because there is no reason for them to be on. Dark mode works great in these sort of dim conditions. It's really a matter of comfort, not fashion.

As for 'at work', it's been my experience that offices generally aren't very bright. Not even remotely as bright as daylight, nor even as bright as electric lighting could be. Those overhead florescent bulbs produce a harsh shade of light.. but not all that much of it. That's why people squint or put on sunglasses when they walk out of office buildings during the day.

I'm talking about people in well lit environments who don't care how well or not it's lit, they just use dark mode because dark mode.