I think OP was referring to the sudden obsession with dark mode (in the past year or so). It's not like screens and night time are new things, so what changed in the past while to induce this dark mode craze?

I think it correlates with the "blue light" studies that were released lately. As far as I know they weren't conclusive, but both Microsoft and Apple implemented Flux-like screen dimming, dark mode seems like a logical step.

Personally I prefer light themes, I just turn the lights on so that I don't sit in the dark.

Exactly. Seems all of a sudden every app is releasing their dark mode that they claim “everyone has been long waiting for”.

Oh, that's simple. Because Apple did it.