I actually think the dark theme looks nice and the colors compliment each other very well. If you wanted to make a small change to add a little flair, maybe add a gradient to the title bars, or something similar to spice it up, while still keeping the theme fairly simple.

As far as content goes, you seem to be doing well. The boards cover a very diverse range of topics and from first glance, you have a decent amount of threads in each board for members to respond to. Is there a way to also display board descriptions to the main page though? I know your board titles are fairly self-explanatory but it's still nice to have the details of what is contained in that board before you actually click on it.

Another small suggestion, you could add a section on the main page that shows how many members have been online and active that day, just to give new potential members a feeling of how active the forum is before joining. This further encourages them to sign up, by seeing how active it is beforehand.