Readings
This is a really great article because it breaks down what kind of work and time actually go into site design, and how it is comprised of different parts - graphic design, information architecture, CMS framework and then porting the design to the framework. Plus...you want and idea of what to charge for your new found skills, right? :)
WordPress
Joomla
Drupal
This tutorial refers to theming in Drupal 4.7 and Drupal 5.0. Theming is significantly different in Drupal 6.