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The Drupal.org site has a great deal of information on how to theme for Drupal 6. Here is an overview with links to the longer articles.
If you have never themed Drupal before, you are probably wondering about what is involved in the process. Theming for Drupal requires an understanding of:
- Proper front-end web development: (x)HTML + CSS
- The Drupal terminology / theme content tags
- PHP knowledge is a plus, but not necessary
To sum it up, a simple Drupal theme is a proper (x)HTML + CSS layout, with embedded Drupal tags through the use of basic PHP code. In action, these Drupal tags get replaced by the content of the site.
Through themes you can also override the front-end code and presentation of output, giving you fine-grain control over the delivery of content.