Last year, I wrote with the help of my colleagues, a new theme for Drupal 7: Atomium. That theme needed to break with the habits from the past and implements new concepts while giving more flexibility to the end-user. As those concepts were pretty new for most of the people using it, I’ve been asked to give training to some teams.
The Drupal 7 theme layer has, and sometimes still is, been a nightmare to understand for me. In my modules, I’ve always tried to avoid this dark corner that are the theme hooks and inspire my code from what I see somewhere else. Until now, it helped me so far pretty well, that’s the beauty of the Open Source.