Are you an elite Drupal themer wanting to add custom settings to the /admin/build/themes/settings/yourtheme page? The Theme Settings API module was written for you, baby!
About the Project
Without this module, it was impossible (in Drupal 4.7 and Drupal 5) for some themes (like PHPTemplate-based ones) to add settings to the theme settings page without coding a module. There needed to be an API to facilitate that, but before this project, no one had successfully implemented it (See issues 54990, 56713 and 57676.)
The Theme Settings API project created a fully functioning implementation of a custom theme settings API for Drupal and then championed the addition of the updated API into Drupal 6.
Themes which use the Theme Settings API include
Theme Settings API in Drupal 6!
The custom theme settings functionality has been added to Drupal 6! See the Theme developer’s handbook for more information.