" Here's a JavaScript stylesheet picker that puts a select dropdown on the page.
altstyle.js : Steve Fulton : 1999.11.22

Select alternate stylesheets. Adds a select dropdown to pick which style to apply to document. Document is loaded with persistent and default style sheets applied. Selector disables default (but not persistent) style sheets and enables alternate stylesheets. Multiple stylesheets can be used in a style, but all sheets with the same title should be grouped.

Place this script in the body where you want the selector to appear.
There are no HTML tags other than <select> and <option> for the style picker."

"Mozilla/NS6: Use menu ( View ->Use Stylesheet )
Gecko-based browsers have trouble with accessing the styleSheets object before the document is finished loading. However, they have an user interface that allows alternate stylesheets to be selected. So the select control is only written in IE."

I've built this demo of the script using the core style sheets from w3c

NOTE: the script builds the select autmaticaly from the title=values of
<link rel="stylesheet" type="text/css" href="default.css" title="default">
<link rel="alternate stylesheet" href="http://www.w3.org/StyleSheets/Core/Chocolate" type="text/css" title="Chocolate">

w3c reference on external style sheets