The style is clean and smooth and easy to navigate.
Now, some people will come in and say "it's just a black/grey version of the default vb style", which in a way is correct. However, my site has
27 styles and every single one of them is essentially just a different coloured version of the default vB style, albeit with many tweaks.
So my point is, don't feel you have to reinvent the wheel...keep it simple. Unless you run a site for hardcore gamers, coders, designers etc then you don't need anything other than neat, tidy, easy to navigate simplicity.
I'm not sure whether some red would improve it or not. The monotone appearance actually gives it quite a neat look in my opinion, whilst remaining nice, light and airy. You could experiment with this though and see how it looks, as black and red styles can look nice if done well.
I like the dashed border round the guest message....I have a similar thing on my site. It makes it stand out well without being intrusive. I would customize the welcome message a bit more though....for instance, the default message tells me I can use "all of the board's features" if I register...but what are they? I know *I* know, but a casual visitor wouldn't, so list a few of the main benefits. That box is, essentially, a marketing document. Use it to full advantage and really sell your site with it. Also remember Google will probably pick the start of that text up and use it in the description in search results. So if done well it can really improve your rankings!