A forum can have additional themes for you to choose from, allowing you to view the forum in a style that suits you more than perhaps the default theme on the forum. When you choose to view the forum in a different theme, you will view the forum with the theme you chose whenever you log in to the forum in the future.
To get started, click the Profile button typically found in the top menu bar of the forum, and then on your profile page click the Edit Profile button. You'll be taken to the Edit Profile & Settings page. On this page, click the Settings tab and you will see the option for Forum Theme as the third option listed on the page. Click on the drop-down menu provided an you'll be presented with the names of the different themes to choose from. Click on the name of a theme that interests you most and then click the Save Account Settings button found at the bottom of the page. When this is done, you should be viewing the forum with the new theme that you have selected.