Theme instructions
Some easy instructions on how to set up and maintain a theme:
Themes are in Site building » Themes. There are four default themes. Switch to another theme by marking both the check mark box and the default button.
Add other themes by going to http://drupal.org/project/themes or the http://themegarden.org/. Make sure that you choose themes that are available in your version of Drupal, that is v 6.x (main version 6 with all its updates).
To use a new theme, download it. If you are in the themegarden, click on the link to the "project" page for that theme and download it from there. Then save it, extract it (Tugzip), and copy the extracted directory to the sites/yoursite.com/themes/contrib/ directory. Upload the entire theme directory into the corresponding server address. Then it will be immediately visible in your site themes section, and can be used.
Note: some themes are not perfect and do not show content properly. I recommend not to use them and delete them.