Zen is a powerful, yet simple, HTML5 starting theme with a responsive, mobile-first grid design. If you are building your own standards-compliant theme, you will find it much easier to start with Zen than to start with Garland or Stark. This theme has LOTs of documentation in the form of helpful code comments in its PHP, HTML, CSS and Sass.
If you are running Drupal 7.x and intend to work with Zen 5.x, then you will need the Sass and Compass tools. These are not installed by default on most Linux distributions (and certainly not on Windows). You probably will find this page helpful for guidance on obtaining Sass and Compass.
Besides this documentation, additional resources can be found…
- Zen project homepage
- Basic Theming with Zen and CSS in Drupal 7 (10 minute video)
- How the Zen Theme is Structured (5 minute video)
- Drupal School: getting started with Zen (20 minute video) (Drupal 5.x and 6.x only)
- Drupal 7: Setting Zen Column Widths (15 minute video)