If you have only one style available, you will need to install another style. In the Admin Panel, click the styles tab at the top of the page. There will be a list of styles that have been uploaded to your forum. If any are not installed, you may install them by clicking the install link. If there are no styles listed, you will need to get a style and upload it. There is a selection of styles at
http://www.phpbb.com/styles/ For information on how to install a style,
How to install styles on phpBB3