The base Zen theme is designed to be easily extended by its sub-themes. You shouldn't modify any of the CSS or PHP files in the
zen/ folder; but instead you should create a sub-theme of zen which is located in a folder outside of the root
zen/ folder. The examples below assume zen and your sub-theme will be installed in
sites/all/themes/, but any valid theme directory is acceptable (read the sites/default/default.settings.php for more info.)
IMPORTANT NOTE: In Drupal 7, the theme system caches which template files and which theme functions should be called. This means that if you add a new theme, preprocess or process function to your template.php file or add a new template (.tpl.php) file to your sub-theme, you will need to rebuild the "theme registry." See http://drupal.org/node/173880#theme-registry
Drupal 7 also stores a cache of the data in .info files. If you modify any lines in your sub-theme's .info file, you MUST refresh Drupal 7's cache by simply visiting the Appearance page at admin/appearance.
This documentation is specifically for people who use Drush. You can take advantage of the
drush zen command to automatically generate a sub-theme.