The default Drupal installation comes bundled with a few themes that you can change from your administration area.
1. From your admin panel, go to Administer -> Site building -> Themes.
2. Once you’ve found a theme, select the Enabled and Default check box and radio button.
Uploading a New Theme
1. If you would like to download and install other themes, more can be found at The Official Drupal Theme Resource.
2. After you’ve downloaded a theme file (most will be a .tar, .rar., or .zip file) extract it to it’s own directory and then upload that folder to the /themes directory of your Drupal installation.
3. You should now be able to go back to Administer -> Site building -> Themes and see the new theme listed.