Zen theme & sub themes for Drupal
If you are building your own standards-compliant theme for Drupal, start with the Zen theme. Its an easy and clean theme with great documentation both included and available online. It has the support of a large community of developers and users. The Zen theme is meant to be a very flexible standards-compliant and semantically correct XHTML theme that can be easily modified through CSS and Drupal’s template system. I have found it works well alongside the CSS Injector module.