ijmccallum

This theme demo / theme switcher plugin will add a bar to the top of your WordPress site listing every theme you have installed. It gives visitors the ability to switch between themes and try out their look on your live site.
After installation it requires no set up, it just works.
If you would like to see any extensions to this theme switcher please don’t hesitate to contact me from my website or post something in the support section. And, if you like it – please leave something nice in the review section! The more popular this becomes the more likely I’ll be to work on giving it lots more extra awesome.
<h4>Why another theme switcher?</h4>
I built this theme switcher primarily for myself. There are a number of other theme switchers out there but they all require visitors to click on a drop down menu then select a theme to demo or have some other complication and none of them had the clean, simple, minimal way of switching that I was hoping for in a theme switcher. So I built this theme switcher to give visitors an instant list of all the themes I hope to develop which they will be able to demo in a single click, also it doesn’t need any set up so it’s very easy!
So, I imagine most of the people interested in this theme switcher would be theme developers but if there are any people who are really indecisive about how they want their website to look, I guess this could provide them with an interesting solution to the website design problem. Let me know if you can think of any other uses for a theme switcher!

IJM Theme Switcher Bar

Add a theme switcher / theme demo bar to your site. Allows users to switch the theme they see on your site.

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.

IJM Theme Switcher Bar vs York IE – Theme Switcher