Editing the theme's CSS file..

Hi, I am new to WP and trying to figure out how themes work. As I go through various threads, people say editing CSS happens by going to Appearance->Editor in dashboard, and then editing style.css file. In my style.css file (I am using theme “Responsive”) there are only some commented out information (between /* */ tags). The real CSS stuff is in core/css/style.css, but there’s no access to this folder through the dashboard editor. Am I doing something wrong? Thanks for any help.