Hi Red and I'm sorry that no one replied to this back when you posted it.
I was working on a dark theme for the forum back when we switched to the current skin we're using. It's not that there isn't a dark SMF theme - there's lots of them. The issue is that the ultimate profile system that FCF uses isn't part of the original forum, so that portion isn't themed along with everything else. That means that switching to a new theme without modifying it will cause the profiles to be a mess.
For this current theme we're using, it took me a couple of weeks to figure out how to get the profiles to look right, and since I don't have admin access, it was basically me pasting code to BigDevil and having him try it out on a separate copy of SMF forum, one we no longer have access to.
I might ask him to add a dark theme option just to see how bad the profiles look. Maybe we'll get lucky. But it won't switch based on day or night. Users will have to select that theme in their profile, and they keep it until they pick a different one.