Yes, it's such an obscure feature that many of the people who enable it don't even realize it... Ever since I added dark mode to gwern.net, I've been getting or seeing complaints about how my website is broken or 'defaults to black-on-white' (it doesn't). :(
I've had a similar thing where people were complaining our app was in English (it was for a Dutch company), apparently people get confused about the preferences they themselves have set.
In all seriousness, because people keep not realizing or forgetting, we're adding a 'theme switcher' little widget. I dislike the need, but lots of people genuinely like dark mode, and this seems to be the best compromise.