Start with our own theme - overview of the info file
Regions
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Demystifying drupal 7 theming
Software
Drupal Theming with CSS Frameworks (960grid)
Technology
Drupal Theming
Documents
Drupal 7 theming with Template Field Variables
Forensic Theming for Drupal
Introduction to Drupal (7) Theming
The History and Future of Drupal Theming.
Drupal 960 grid system based theming
Design
Drupal theming intro
Drupal 7: Theming with Omega
Drupal 7 Theming - Behind the scenes
Education
Theming Drupal 6.x Forms
(some) Drupal Theming by Ryan Price
Drupal 8 theming
Drupal 8 theming deep dive
Internet
Drupal Theming Limitations
Forensic Theming - Drupal Theming Design, UX and Theming Emma Jane Hogbin @emmajanedotnet
Drupal 8 Theming Done Right (European Drupal Days 2015)