ZEN'S CSS FILES --------------- All of the files in this folder are CSS files that are generated from the Sass source files. In older versions of Zen, you could modify the CSS files and ignore the Sass files. But when we converted Zen to using component-based styles, it was clear that it is too hard to write CSS when Drupal makes it hard to change the markup. You should: - ignore these files - modify the Sass files instead - learn how to generate the CSS from the Sass. See your sub-theme's README.txt.