We have found, though, that this perfunctory approach to theming misses out on some very powerful benefits.
Drupal, our usual platform of choice, inherently provides tools and conventions dedicated to theming. These tools inspired the realization that theming is most powerful when treated as a critical process separate from and as important as the processes for creating content and presentation.
In this approach, theming involves more than combining the content and presentation. The three-tier approach adds a “theming” tier to content and presentation. Just as presentation and content have dedicated roles associated with them (designers and front-end developers for presentation and back-end developers for content), the theming approach we advocate requires a themer role for successful execution.
The themer not only joins together the content and presentation, but also to oversee the establishment, enforcement, and implementation of some important rules.
Rule #1: Separation of content and presentation
Rule #2: Define and use globals
Rule #3: Use a cascading hierarchy