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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.

thom4

fabrizim

<a href="http://lesscss.org" rel="nofollow ugc">LESS</a> is a templating language based on top of CSS. It provides numerous enhancements to speed up development and make its maintenance easier.
Theme developers can even bundle the plugin without worrying about conflicts: just include the special <code>bootstrap-for-theme.php</code> and read its instructions.
<h4>Features</h4>
<ul>
<li>Variables</li>
<li>Mixins (inheritance of rules)</li>
<li>Nested Rules (write less, do more)</li>
<li>Accessors (inherit a value from a specific rule)</li>
<li>Functions (logic operations for dynamic results)</li>
</ul>
The plugin lets you concentrate on what you need: coding CSS. Everything else is handled automatically, from cache management to user delivery. 
Seriously.
<h4>Documentation</h4>
Advanced topics on how to use the plugin API are <a href="https://github.com/oncletom/wp-less/tree/master/doc" rel="nofollow ugc">available on the Github project documentation</a>.
<h4>Requirements</h4>
The sole requirement is to use WordPress API and LESS convention: the <code>.less</code> extension.
Minimal Requirements: PHP 5.3 and WordPress 3.2. 
Relies on: <a href="http://lessphp.gpeasy.com/" rel="nofollow ugc">Less.php</a>, <a href="https://wordpress.org/extend/plugins/plugin-toolkit/" rel="ugc">plugin-toolkit</a>.
Notice: in case you’d like to drop the usage of this plugin, it’s safe to do it. You will just need to convert back your stylesheets to CSS.

WP-LESS

Implementation of LESS (Leaner CSS) in order to make themes development easier.

Development Theme vs WP-LESS