Your cascading style sheets are where you will change the appearance of your site. Don't like the back ground color, text size or the overall appearance of you pages? All of this is controlled in the css file.

You can edit the stylesheet via Admin Panel -> Themes -> Manage Templates -> Stylesheet, or the /templates/styles/default.css file offline in your favorite HTML/CSS editor. The stylesheet has been simplified for you to contain everything you need to change all the colors and fonts, with no cruft. All the detailed cruft is in the base.css file in your theme's directory, but you don't need to edit that because you can override any of its rules in the regular stylesheet (since the regular stylesheet is called later than base.css in the HTML). Add just what you need and not what you don't to keep your style simple and readable.