The bottom margin for your forum category buttons could be a bit larger, you should add a bottom border to your postbit, the 'New Reply' button should match your theme, and your forum category headers should have a lighter shade of black on the bottom border (personal preference).
This is quite nice. Most MyBB themes are shit (the ones for free anyway) but this pretty decent.
However, I would say vertical-align the navigation and maybe make the font stand out a little more. I wouldn't go with a brighter background as I take it you're making this a dark theme?