Theming Drupal
A site's 'look and feel' in geek speak is called a Template, a Front End Overlay, a User Interface (UI) and the User Experience (UX), or in Drupal, the "Theme." You probably already know that ...
This document offers some guidelines on when you should start theming a Drupal project, just how varied Drupal themes can be, where to find 'free' themes, and steps you can take to reduce your theming costs.