Not sure about Windows or macOS, there users probably stick with default setting anyway, but here on GNOME I have quick access to "Dark Style" toggle in quick settings, just one click away.

We could start a long discussion, should websites just follow system settings, or add their own custom controls, where you have different settings for each website. Along with dark mode switching, we should consider accessibility controls, and cookie settings too.

From a developer standpoint I'd prefer to reflect system settings.