A comprehensive guide to themes and theme development can be found on the Drupal website at http://drupal.org/node/509. Themes generally consist of three layers:
A PHP template engine
An xHTML template
Styles in the form of CSS
It is recommended to start creating our theme from an existing Drupal theme; the themes that are recommended to use are Zen or Bluebreeze. The theme that we are going to create is only a simple one-page theme, which will use default styling provided by each module.
Drupal 5 Themes by Ric Shreves, ISBN 978-1-84719-182-3 published by Packt Publishing, it is a book published specifically for creating themes ...