They can? Is this a recent thing? I remember wanting to detect the background colour years ago, and not finding any way to do it.

It's not recent, and most terminals support it. You send an escape sequence to the terminal, and get back a sequence that tells you the exact background color.

Huh, indeed. I still can't find much information about this, but this page is very informative: https://jwodder.github.io/kbits/posts/term-fgbg/