It also looks rather large, do you expect hundreds of posts? If not, you may want to reduce the numbers of categories so that people actually read the posts. People get put off by too wide a scope (and they don't know where to post).
Agree a homepage would be good, explaining the purpose of the forum etc.
And even if you just change the background colour and replace the "simple machine" logo with your own, it needs something different.
Good start though.