Introduction

One of the more prevalent adages with respect to Drupal development and theming is:

Do not hack core!

Modules, themes, and other files which come with a stock Drupal installation should never be edited directly. In other words, we really should not need to modify anything outside the sites folder which is designed to contain all our changes and customizations. The reasoning behind this is that most, if not all, aspects of core are accessible and modifiable through a clean and non-invasive process using Drupal's APIs. Therefore, hacking core modules and themes to get things done is almost always unnecessary and ill-advised.

Another reason why directly editing core modules and themes, or for that matter, even contributed modules and themes, is that whenever an upgrade of Drupal or said modules and themes takes place, we will very likely be overwriting the changes we have made, or at the very least, make the upgrade a trying exercise.

With respect to themes, let us take the example...