Worth noting is that "prefers-color-scheme" works in all major OSes at this point, mobile and desktop. If your site already has a dark theme, there is no reason at all not to be integrating it this way. Even if you don't have a dark theme already, it's a huge value-add..... (@dang?)
It's funny to me that dark themes seem to be the preferred theme type in online discussions, because in practice, most people I know prefer a light theme on a monitor set to high contrast and relatively low brightness in a well lit setting. I too prefer that, as it seems to reduce (eliminate?) my eyestrain after hours on the computer.
I think that's the key point there. Light themes are good in light, but bad in dark situations, and vice versa. "Most" people prefer light, because they use their computer mostly in the light. Many programmers etc. prefer dark because they use their computers late in the dark a lot.
These are basically two "modes" of use, but software has been treating this as a preference, rather than a mode switch.
Through a set up of PowerShell/Bash scripts, I now have global dark/light mode switches on all my devices and I feel like this is the first time I can really use them effectively in all situations. Light when there's light, dark when there isn't. I even switch on the train as it goes through a tunnel. IMO, this is how it should be used, not as a set-and-forget preference.