WordPress theme switcher

This has to do with .htaccess rather than wordpress itself.

You could just add a row to your htaccess files.

RewriteRule ^(.+)/theme-(.+)$ $1?themedemo=theme-$2%{query_string}

$1 is argument one, the first appearance of (.+) and is the page name
$2 is the second argument, the second appearnce of (.+) as in the theme name

so if you write:

example.com/about/theme-thesis it should get the content from example.com/about?themedemo=theme-thesis

try it out!