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.

Ermald Billa

Improve Accessibility – Dark Mode Switcher for WP adds a professional dark/light mode toggle to your WordPress website.
Features:
<ul>
<li>Universal dark/light mode toggle</li>
<li>localStorage support for user preferences</li>
<li>Modern responsive design</li>
<li>Full WordPress security implementation</li>
<li>JavaScript API for developers</li>
<li>Mobile-friendly</li>
<li>Translation-ready</li>
</ul>
Security:
The plugin implements all WordPress security standards including nonces, sanitization, escape output, and permission checks.
Developer API: 
 `javascript 
// Get current theme 
window.iadsAPI.getTheme();
// Change theme 
window.iadsAPI.setTheme(‘dark’);
// Toggle theme 
window.iadsAPI.toggleTheme();

Improve Accessibility – Dark Mode Switcher

A secure WordPress plugin to add a universal dark/light mode toggle with localStorage support.

JP Theme Switcher Bar vs Improve Accessibility – Dark Mode Switcher