I think pretty much everywhere I use the dark style option if it is available. Even my own users begged to have one but for the past 1 year I have been crazily lazy . I even asked about a very big forum's admin to have his dark theme options and he was very kind of sharing it with me. But I didn't have the time to implement it yet. Now, with this I don't have to do the dirty work anymore .
I started configuring a dark "default" style for XF 2.1 back when I was in the planning stages of an upgrade, but ended up going with a third party theme instead.
Yet creating a dark theme really isn't too difficult once you get used to XF's advanced template and styling syntax.
The real party trick would be to have XF detect if the visitor's device is set to a light or dark theme, then pick either light or dark theme automatically. One of my forums uses a theme that places a lightbulb icon up near the member's search bar, and clicking on that toggles between light/dark options.