To avoid obscuring CSS added to the page by Drupal or a contrib module, the Stark theme itself has no styling, except just enough CSS to arrange the page in a traditional “Header, sidebars, content, and footer” layout. See the layout.css file for more information.
The Drupal 6 version
The Stark theme is now included in Drupal 7 core. This is an exact copy backported to Drupal 6.
The Drupal 7 version
This contrib version of Stark is an exact copy of the one in Drupal 7 core with one exception. This version now uses a responsive design that matches a Drupal 8 proposal to make Stark use a responsive layout.