Drupal is content management system supplied with a default theme, some optional themes, and access to thousands of downloadable themes. Some of those themes are good, some are fun, and some are accessible. Here are some of the best choices.
When you register and login, you can try out some of these themes on this site. There is an option in Drupal to let users choose their own themes. The Drupal administrator can download and test any mixture of themes. The themes can be individually switched on and off plus given individual settings. As Administrator, you go to Administer » By module » System » Configure permissions then switch on select different theme for authenticated user. A logged in user can go to My account then Edit and see a theme configuration list.
drupal.org where you download the theme. Some themes also have documentation at drupal.org.
The first thing to look for in a theme is compliance with the standards for XHTML and CSS. If the output from a theme is not compliant, you will have endless errors, both obvious and mysterious. A standards compliant theme also indicates the theme developer is experienced and focused on the theme. Start by browsing the themes under D-theme XHTML 1.0 strict.