It looks like the class "res-nightmode" is applied to the body when you enable the dark theme. That can be used to tweak font and background colours accordingly. I quite like these theme tweaks, but I'm not sure about taking the forum header colour, I don't really like the forum theme too much in the first place
The darker styles seem nice to me. I'm using a few purples in the WordPress stuff simply because they compliment the header space image. Maybe that would be something to bear in mind too.