The Appearance overlay is where themes are located. The overlay has two tabs: List and Settings. List lets you choose a theme and Settings lets you adjust your theme.
To set the default theme:
- Click Appearance, then List.
- Click Set default next to the theme you want.
Some themes may need to be enabled before you can use them. Click Enable and set default next to the theme you want.
At the very bottom of the List overlay there is a popup menu to change the administrative theme. Most people elect to leave what’s already chosen, as the default presents a simple, clean interface for administrative tasks. Changing the administrative theme should only be done by advanced users, as some changes can cause unexpected results for those new to Drupal.