Important Drupal Theming Concepts
How to determine which method to use when modifying output
A conceptual introduction to the Features module for exporting configuration options as code
How to decide whether to use a module or a theme for your modifications
How to decide when to use a theme function or template file
How the theme registry works
How to find out where any piece of output comes from
What view modes are and why you would use them
How to adjust node view modes through the user interface
What are field formatters and how to use them to modify field output
How coding standards apply to the theme layer, and how to add comments to template files
Template file coding standards and conventions
How to apply CSS coding standards
Theme troubleshooting and what to do when you don't see your theme output changes applied
How to troubleshoot CSS problems in your theme