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.

Seth Smigelski

Slim Mobile Theme Switcher is a developer-friendly, lightweight solution for sites that need a dedicated mobile experience without the overhead of a heavy mobile plugin. This plugin targets mobile handhelds, ensuring tablet users may still enjoy your full desktop-grade layout.
<h3>Key Features</h3>
<ul>
<li>True Mobile Detection: Uses refined regex to separate small-screen phones from tablets (iPads/Android tablets).</li>
<li>Manual Overrides: Allow users or testers to force a view using <code>?theme=handheld</code> or <code>?theme=active</code>.</li>
<li>Persistent Choice: Option to remember a user’s manual theme choice for 30 days via cookies.</li>
<li>No Bloat: Zero front-end CSS or JS added by the plugin itself.</li>
<li>Developer Friendly: Built by a developer for developers.</li>
</ul>
<h3>How It Works</h3>
The plugin uses a precise, two-step detection process to ensure the best user experience: 
1. Exclude Tablets: High-resolution devices (iPads, Android tablets, Kindle Fire) are served the Desktop Theme to take advantage of their screen size. 
2. Detect Phones: Handheld mobile devices (iPhone, Android Mobile) are served the Mobile Theme. 
3. Manual Choice: If a user manually switches via URL, their choice is honored for 30 days via cookies.

Slim Mobile Theme Switcher

Serve a mobile theme to phones while keeping desktops/tablets on the primary theme. Lightweight mobile theme switcher with modern device detection.

JP Theme Switcher Bar vs Slim Mobile Theme Switcher