Chapter 9. The Theme System
Changing the HTML or other markup that Drupal produces requires knowledge of the layers that make up the theme system. In this chapter, I'll teach you how the theme system works and reveal some of the best practices hiding within the Drupal 7 core. Here's the first one: you don't need to (nor should you) edit the HTML within module files to change the look and feel of your site. By doing that, you've just created your own proprietary content management system and have thus lost one of the biggest advantages of using a community-supported open source software system to begin with. Override, don't change!
In Drupal-speak, themes are a collection of files that make up the look and feel of your site. You can download ...