How'd I do that?

More importantly: Why? I used that little trick to demonstrate the power of CSS, a more advanced way to construct websites that separates style from content.

CSS is a step beyond HTML, and enables me to create websites that you can update yourself and still keep the great look we design for it intact.

CSS sites are also standards compliant (learn more about W3C standards), accessible, and search engines love them.

Technical Explanation

When you clicked the [red/blue/org] links nothing changed but the CSS page that contained all the page colors, text (style/color) and positioning information. The page that contained the content was exactly the same page. It did not change at all.