If your feeling festive and want to change the appearance of the forum, click the Profile link in the main menu. This will open up your profile area where you can access the member sub menu and all your options.
In your profile area, on the left above where it says Summary w/the little girl icon, is where the member sub-menu is. Navigate: Modify Profile > Look And Layout.
Next to where you see Current Theme click (change). Here you can preview and/or choose which theme you want to use. Do the same thing if you want to change it back!