I see it in lots of products and kind of ignored it, but is this a priority for users?
On OLED screens like my phones, black reduces energy use. On regular screens, dark mode subjectively seems to make things easier on my eyes. I also use f.lux and related technologies. I also sometimes use yellow-tinting screen glasses. I have no idea if any of this is scientifically valid, but at this point I can feel my eyes decline significantly as I age. My particular vision is 20/20 but one of my eyes is much worse and this contributes to a lot of eye strain. I also have pretty chronic headaches.
Since my entire ability to make money hinges on computer screens, I will take whatever nonsense option seems, subjectively, to make things easier on myself or preserve my longevity.
GitHub and HN are probably the two things I use most that do not support dark mode natively. My sense is that as far as feature requests go, dark mode typically does not tie up resources that otherwise would be allocated to more functionality-based features. GitHub doesn't need to provision more servers, for instance, in order to implement dark mode. HN already has colour customization, we need only to be able to customize a few other colours to allow dark mode natively.
I recently worked my way down feature requests enough to reach dark mode for a site I run and users were ecstatic, moreso than they were for big, new features that'd been requested thousands more times. I don't use it personally, but I think it's a "feel good" thing when you have the choice of dark or light mode.