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.

Nashwan D

Conditional Themes is an API to switch the themes on certain conditions.
<h4>Usage</h4>
Write an another plugin file and use the Conditional Themes API as the example below:
<pre><code>add_action('plugins_loaded', 'my_conditional_themes_setup', 100);

function my_conditional_themes_setup()
{
 // Switch to Twenty Eleven theme if the visitor use Internet Explorer.
 Conditional_Themes_Manager::register('twentyeleven', function() {
 global $is_IE;
 return (bool) $is_IE;
 });

 // Switch to Twenty Thirteen theme if the user has administrator role.
 Conditional_Themes_Manager::register('twentythirteen', function() {
 return current_user_can('administrator');
 });

 // Switch to a custom theme if the visitor use a mobile device.
 Conditional_Themes_Manager::register('mobile', 'wp_is_mobile');
}
</code></pre>
Another example, With enabling persistent mode.
<pre><code>add_action('plugins_loaded', 'my_conditional_themes_setup', 100);

function my_conditional_themes_setup()
{
 // Enable the switcher persistent mode.
 Conditional_Themes_Manager::set_option('persistent', true);

 // Switch to Twenty Sixteen theme when we being on 2016.
 Conditional_Themes_Manager::register('twentysixteen', function() {
 return (date('Y') == 2016);
 });

 // Switch to Twenty Fifteen theme when the site reaches 500 post.
 Conditional_Themes_Manager::register('twentyfifteen', function() {
 return ((int) wp_count_posts() > 500);
 });
}
</code></pre>
Note: You can use <a href="https://wordpress.org/plugins/code-snippets" rel="ugc">Code Snippets</a> plugin to add the code snippets to your site.
<h4>Contributing</h4>
Developers can contribute to the source code on the <a href="https://github.com/nash-ye/WP-Conditional-Themes" rel="nofollow ugc">Github Repository</a>.

Conditional Themes

Keep up the good work. Plugin exactly doing what it should do. Perfect :)

Awesome Plugin. Neat and Accurate

A simple API to switch the themes on certain conditions.

JP Theme Switcher Bar vs Conditional Themes