Registered users can change the look of the forum. This will only change it for the user. To change the appearance of the forum, you must be logged in. Then go to the "Profile" button at the top of the page. Towards the bottom of the page is the section, "Preferences". Next to the "Board Style" area, click on the arrow to reveal the names of other styles. Then select one to try out.
When you are done, hit the submit button at the bottom of the page. When you go back to the forum, you will see your change. If you don't like it, just go back to your Profile page and change it to something else.