Arya Themes

Allows users to choose and preview all WordPress themes installed without 
activation or deactivation for demonstration purposes.
<h4>Usage</h4>
<pre><code>https://example.com/?theme=slug-theme
https://example.com/?theme=slug-theme&child=slug-child-theme
</code></pre>

Arya Switch Theme

Works like a charm, perfect for theme developers.

Absolutely Brilliant !

Allows users to choose and preview all WordPress themes installed without

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.

Arya Switch Theme vs York IE – Theme Switcher