FYI, for visually impaired users, Windows has offered dark high contrast themes since about forever. I say visually impaired because these themes use some weird colors that unimpaired users likely won't appreciate.

These high contrast themes also make web sites dark automatically, at least in Firefox and probably Edge (haven't checked Chrome).

Additionally, Windows 10 has a Dark mode that looks similar to what Apple is bringing to macOS, but I don't think that affects web sites at the moment.