Josh Pollock

Use this plugin to create a demo site for your themes, or use it to allow users to customize the presentation of your site.
The JP Theme Bar Plugin adds a theme switching bar to the bottom of your site, perfect for theme preview sites. The settings page for the plugin lets the end user choose which themes to add, as well as set the colors for the theme bar. You can see it in action on this site.
New in version 0.0.3 -> Themes will update theme mods when changing themes allowing for individual theme settings to be previewed properly.
This plugin is based on the <a href="http://iainjmccallum.com/wordpress/live-demo-theme-bar/" rel="nofollow ugc">IJM Theme Switcher Bar v2.0</a> by <a href="http://www.iainjmccallum.com/" rel="nofollow ugc">Iain J McCallum</a>
IMPORTANT- The plugin <a href="https://wordpress.org/plugins/theme-test-drive/" rel="ugc">Theme Test Drive</a> by <a href="http://www.prelovac.com/vladimir/" rel="nofollow ugc">Vladimir Prelovac</a> must be installed and activated in order for the theme switching to work.

JP Theme Switcher Bar

Adds a theme switcher/ theme demo bar to the bottom of your site to allow users to switch the theme they see on your site.

husterk

The User Level Themes plugin allows a site administrator to select different themes for users with and without administrative 
access rights. This feature can be used to safely test a new theme while redirecting visitors to a known working theme. This plugin 
utilizes the [Wordpress Plugin Framework] (http://www.doubleblackdesign.com/categories/wordpress-plugins/wordpress-plugin-framework/ “WordPress Plugin Framework”) to 
provide a standard administration interface and allow for simplified plugin management.
<h4>License</h4>
This User Level Themes and WordPress Plugin Framework are being developed under the GNU General Public License, version 2.
<a href="http://www.gnu.org/licenses/old-licenses/gpl-2.0.html" title="GNU General Public License, version 2" rel="nofollow ugc">GNU General Public License, version 2</a>
<h4>Support User Level Themes Plugin Development</h4>
Help support development of the User Level Themes plugin and WordPress Plugin Framework by making a donation.
<a href="https://www.paypal.com/cgi-bin/webscr?cmd=_xclick&business=husterk%40doubleblackdesign%2ecom&item_name=Wordpress%20Plugin%3a%20User%20Level%20Themes&no_shipping=1&no_note=1&tax=0&currency_code=USD&lc=US&bn=PP%2dDonationsBF&charset=UTF%2d8" title="Donate to Double Black Design" rel="nofollow ugc">Donate to Double Black Design</a>
<h4>Project Location</h4>
The latest version of the User Level Themes plugin can be found on the WordPress Extend website. A link to this site is provided below.
<a href="https://wordpress.org/extend/plugins/user-level-themes/" title="Download the User Level Themes Plugin" rel="ugc">Download the User Level Themes Plugin</a>
The latest version of the WordPress Plugin Framework can be found on the WordPress Extend website. A link to this site is provided below.
<a href="https://wordpress.org/extend/plugins/wordpress-plugin-framework/" title="Download the WordPress Plugin Framework" rel="ugc">Download the WordPress Plugin Framework</a>
The WordPress Plugin Framework project can be found on the Google Code website. A link to this site is provided below.
<a href="http://code.google.com/p/wordpress-plugin-framework/" title="Wordpress Plugin Framework Project" rel="nofollow ugc">WordPress Plugin Framework Project</a>

User Level Themes

The User Level Themes plugin allows a site admin to specify the site's theme based on the user's access level.

JP Theme Switcher Bar vs User Level Themes