Theme documentation

The Drupal themes provided by hook_theme() are designed to be more configurable and more user-friendly than most standard Drupal themes, with an emphasis on the "little things" that most themes lack. Some examples range from being able to (easily) customize the breadcrumb separator, to being able to set whether or not user pictures are displayed in "teaser" mode, all the way to telling the theme to scan for custom css files so that modifications can be made to the theme without having to hard-code said modifications (because when you hard-code something, the next time the theme requires an update, all modifications will be lost). Not to mention hard-coding is a big no-no. Never hard-code stuff. It may seem convenient at the time, but ultimately it will make things much harder.

NOTE: from this point forward, it's assumed that you're using the Bella Drupal theme, and that the theme itself lives inside the sites/all/themes/bella directory of your Drupal installation.