The CSS is contained in a number of files, so the main CSS file contains only part of it. This is one of the reasons that the original CSS file link is not shown at this time.
Modern browsers have web inspector built into them now-a-days which you can use to identify the selectors that control the styling on page elements. One of the best is the Firebug Add-on for Firefox. It will show the markup (XHTML) on one side of the Firebug pane at the bottom of the browser window, and the CSS on the other side. You can use the inspector (square with an arrow) button to activate the inspector and then hover over the different elements of the page and Firebug will display the markup and the CSS for the element. You can also try changes "on the fly" in the CSS to get things they way you want them, and then copy it and paste it into the CSS edit window in your dashboard. You can get some more information here: http://en.support.wordpress.com/custom-design/editing-css/#css-help .