WPMU DEV Global Theme Switcher

Theme Switcher gives your users the ability to switch between themes from the front end of your site.

It also provides you with the opportunity to preview different theme designs quickly and easily. Plus, it works perfectly for any WordPress, Multisite or BuddyPress site.

Switch From Anywhere

The plugin comes complete with a handy widget that allows you to offer either a list or drop-down choice of themes.
You can also have a theme switcher right in your WordPress toolbar.
One of the coolest things about this plugin, is that it allows previews of different theme designs via nice URLs.

Give Your Clients Options

This a must have for anyone wanting to offer their users maximum flexibility of personalization or to demonstrate to clients different layout options on a WordPress, Multisite or BuddyPress network.