The context matters. Dark themes, where? I guess OS? Or WWW? In terminal and CLI, it was and is the default. The difference has been the color of the text (green, white, grey, yellow). This was during the time of CRT monitors though.

The commercial web popularized white background. Companies like eBay, for example. Although my favourite website back in those days (Webwereld) had some kind of pastel light yellow, IIRC. Marktplaats (popular alternative to eBay, old) also still has this. I suppose the colour is mostly akin to old newspapers, and backlight.

HN also has this colour, by default. Goes very well with the orange (tho I'm biased as that is my fav colour). Though I use a CSS sheet to turn HN into Solarized Dark (which according to the article is voluntarily hurting myself). I'm actually fine with the HN theme, during day. I'd be OK with Solarized Light during day, and Solarized Dark during night. Same with Android and macOS. What worries me is that it ain't going to work fluently for all the programs and settings.