Put two classes (or IDs) in your stylesheet. One of them should have the property:
display: none;
Then use Javascript to write out a div. Have it choose which of the two classes to write out based on what the user selected. (This won't work in all browsers, of course. OmniWeb, for example, doesn't support CSS well.)
This <a href="http://www.alistapart.com/stories/alternate/">A List Apart article</a> explains a pretty good style switcher.