[This Plugin is causing problems with latest version of WP. Please don't download. I'm fixing it ASAP.] MDC Theme Switcher allows to change
MDC Theme Switcher allows to change and preview among available themes of a WordPress from front-end. Upon activation, it adds a sricky bar to front-end with a dropdown list of availabe themes. Admin can specify either a specific theme will be shown in the dropdown or not.
It also enables a shortcode
[mdc_theme_swicher], which can be added anywhere of the WordPress. In a post, page or widget. Even in template files!
It handles with the session issue. So, it shows different outpus to different persons if they select different theme at the same time!