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Let use diferent themes installed for each user role actived.
You can set which theme will be use visible for “administrator” role while logged, or for “editor”, or any other role created. Also, you can set which theme will be visible for rest of visitors, that´s rest of people that are not wp users, just visitors.
It´s very easy, just install it, go to Development Themes Options page, choose your roles/themes and save.
You can use this plugin for things like:
<ul>
<li>
Put your site under maintenance using some other alternative theme for your site, but let the administrator or any other user role be able to see and work a new theme, let´s say an update.
</li>
<li>
You can test some new theme while rest of visitors still see the current one.
</li>
<li>
In fact, you can show a theme for any user role that is actived, default ones, or any other role created by some other plugin. Let´s say one of those role managers available. I´ve tested, and works.
</li>
</ul>
You can also turn Off the plugin, in order to use the actived theme for all users and visitors.

Development Theme

Let use diferent themes installed for each user role actived.

scribu

AppThemes

With this plugin, super-admins can:
<ul>
<li>view any theme on any site, without enabling it</li>
<li>access any site through Admin Bar -> My Sites, without having a role on it</li>
</ul>
Intended for <a href="http://scribu.net/wordpress/the-future-of-multisite.html" rel="nofollow ugc">small Multisite installs</a>.

Hyper Admins

Simplify administration tasks for super-admins.

Development Theme vs Hyper Admins