Changing the HTML or other markup that Drupal produces requires esoteric knowledge of the layers that make up the theme system. The theme system is an elegant architecture that’ll keep you from hacking core code, but it does have a learning curve, especially when you’re trying to make your Drupal site look different from other Drupal sites. We’ll teach you how the theme system works and reveal some of the best practices hiding within the Drupal 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 the biggest advantages of using a communitysupported open source software system to begin with. Override, don’t change!