When we first installed the board, there were only three themes (known as skins on IF forums). Recently I've installed several new ones. To get to them and see what suits you best do the following:
1. Click Profile on the upper toolbar.
2. Once you get to your profile, you should see your avatar and stuff. On the left toolbar of this screen, click Look and Layout Preferences.
3. Once you are there, do you see "Current Theme: [insert theme here] (change)"? If you do, click change. Then, choose a skin/theme.
Any questions?