I'm always amazed when people go through the trouble of adding a light/dark theme switcher rather than just respecting the user's system preference with `prefers-color-scheme`[0].
You can do both. And then you help out people who don't know that they have a meaningful system preference (or maybe all the other sites they visit don't honour it, or maybe their web browser isn't picking it up for some reason).
Like I prefer to read prose and literary website content in light mode while keeping my OS in dark mode.
Admittedly it's a niche use case.