Tom Lynch

Switch Theme allows WordPress Network Administrators to switch all the themes on their WordPress installation to a different theme in only a few clicks.
This is particularly useful if you have large network of blogs which you wish to switch to a new version of your theme.
If you find this plugin useful please <a href="http://supportus.cancerresearchuk.org/donate/" rel="nofollow ugc">donate to Cancer Research UK</a>, and let me know what think you via <a href="http://tomlynch.co.uk" rel="nofollow ugc">my website</a>.
Switch Theme is only for use with WordPress installations with multisite enabled, it will not work on standalone installations or WordPress MU.

Switch Theme

Switch Theme allows WordPress Network Administrators to automatically switch all the themes on their WordPress installation to a different theme.

yorksupport

York IE – Theme Switcher lets editors assign a theme to a single piece of content or preview how that content looks in another installed theme. The site-wide active theme stays unchanged.
Features
<ul>
<li>Assign a theme per post, page, or public custom post type</li>
<li>Settings page to enable theme switching per post type</li>
<li>Preview any supported item with another installed theme in a new tab</li>
<li>Content Theme meta box in the editor sidebar</li>
<li>Front-end admin bar menu for quick theme previews while logged in</li>
<li>Preview banner with an exit link when previewing</li>
<li>Works with subdirectory WordPress installs</li>
</ul>
How it works
<ol>
<li>Edit a post, page, or custom post type.</li>
<li>Choose a theme from the Content Theme sidebar panel.</li>
<li>Click Update to save the assignment, or use Preview with selected theme to test without saving.</li>
</ol>
Visitors see the assigned theme only on that content item. Other pages continue using the active site theme.
Permissions
<ul>
<li>Theme preview URLs require a valid nonce and permission to edit the item or the <code>switch_themes</code> capability.</li>
<li>Assigned themes are visible to all visitors on the front end for that content item.</li>
</ul>
Developer filters
<ul>
<li><code>pagethsw_supported_post_types</code> — Limit or extend supported post types.</li>
<li><code>pagethsw_is_supported_post_early</code> — Control post type support before <code>init</code>.</li>
</ul>

York IE – Theme Switcher

Assign or preview a different installed theme on individual posts, pages, and custom post types without changing the site-wide active theme.

Switch Theme vs York IE – Theme Switcher