Allows you test how a theme looks on your site without activating it.

Noted That:

To install correctly this theme-preview.zip.
Fisrt Download the theme-preview.zip to your computer
Extract/Open theme-preview.zip to Your Computer.
Then, Find readme.txt file inside theme-preview.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "theme-preview.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "theme-preview.zip" Downloaded plugin from your computer, Where you downloaded theme-preview.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "Theme Preview" folder is added on left sidemenu. Now, Click on "Theme Preview" folder.

Noted that: If you do not see "Theme Preview" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about Theme Preview Configurtions and Settings or How to work "Theme Preview" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "Theme Preview" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is Theme Preview
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
Upload the `theme-preview` folder and its contents to your `wp-contents/plugins` directory. Activate in the `Plugins` menu. Visit your site with a special parameter added to the URL Add query variables `preview_theme` and/or `preview_css` to your query string. For example, if you have a theme named “My Theme”, which is installed in your `wp-content/themes/my-theme` directory, add the theme’s directory name to your URL like this: `http://example.com/index.php?preview_theme=my-theme` Sometimes, you create a new look for your site by just making new CSS, but you keep the existing PHP files intact. In that case, you con use the preview_css variable instead of `preview_theme`, or use them both together, like this: `http://example.com/index.php?preview_theme=default&preview_css=my-theme`

Upload the theme-preview folder and its contents to your wp-contents/plugins directory.
Activate in the Plugins menu.
Visit your site with a special parameter added to the URL

Add query variables preview_theme and/or preview_css to your query
string. For example, if you have a theme named “My Theme”, which is
installed in your wp-content/themes/my-theme directory, add the theme’s
directory name to your URL like this:

http://example.com/index.php?preview_theme=my-theme

Sometimes, you create a new look for your site by just making new CSS, but
you keep the existing PHP files intact. In that case, you con use the
preview_css variable instead of preview_theme, or use them both
together, like this:

http://example.com/index.php?preview_theme=default&preview_css=my-theme

site, without, activating, your, looks, test, theme, allows,

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Reffered: https://wordpress.org/