As far as the CMS is concerned, there is no style sheet.
the styles are applied to the pages that display the content, not the to the content itself. using a CMS system separates the content from the design.
In the Power CMS Version included with Design Extender 1.1.5 you can add styles to the styles list by editing the webassist/ckeditor/style_lists/stylelist1.js file.
there are a few example style defined for reference. see this page for more details;