It's possible.
The caveat is that it's global. So If I switch to dark - everyone must then use the dark theme.
While I personally prefer dark theme's, I don't wish to force it on anyone.
I also feel like dark themes on forums have a bit of stigma that make it less then welcoming then a light forum. Just IMO.
I'm happy to consider middle ground though through CSS. If a lot of users here prefer it I will change it