Nilo Velez

System Fonts enqueues a stylesheet that lets you use the native font on each operating system and defines a system-ui css @font-face that you can use on the customizer, your theme’s settings or your own code.
<h4>Usage example</h4>
The Twenty Seventeen theme uses this css selector for the body text:
<pre><code>body, button, input, select, textarea {
 font-family: "Libre Franklin", "Helvetica Neue", helvetica, arial, sans-serif;
}
</code></pre>
We are going to replace it with the default system font. 
In you WordPress site, go to <code>Appearance > Customize > Additional CSS</code> and paste this code
<pre><code>body, button, input, select, textarea {
 font-family: system-ui;
}
</code></pre>
Appart from them font-family, there are other font-properties available:
<ul>
<li>font-style: normal or italic;</li>
<li>font-weight: 300 (light), 400 (regular), 500 (medium), 700 (bold);</li>
</ul>
For example this is the code make an element’s text semibold and cursive:
<pre><code>#my-element {
 font-family: system-ui;
 font-weight: 500;
 font-style: italic;
}
</code></pre>

System Fonts

Enqueues a stylesheet that lets you use the native font on each operating system.

Harvey J

This plugin allows you to preview your themes or to allow users to preview a theme on the front end. 
It renders a different theme for the user on your site without changing the current theme. It also offers a 
download link for the previewed theme provided that the theme is in the “wp-content/themes” directory of your 
wordpress installation. In version 1.2 a theme shopping cart was added to enable you sell your precious premium 
themes. After a user buys your theme he/she will be redirected back to your site and he’ll will be registered using 
the email he purchased with. The transaction is saved and client can now download his theme from the sidebar. 
There’s a catch though, the client must be logged in to download his/her premium theme(s). If all your themes are 
free, no login is required. Users can just download whenever…
Note: this plugins only pulls themes from your <code>wp-content/themes</code> directory, the same directory where zipped files will be stored.
<ul>
<li>To manually trigger a theme preview, type <code>?fet_preview=theme-folder-name</code> at the end of your site url; like <code>http://myawesomesite.com/?fet_preview=my-theme</code></li>
<li>Apply a similar procedure for a theme download:- <code>http://myawesomesite.com/?fet_download=my-theme</code></li>
</ul>
<h4>Features</h4>
<ul>
<li>Front-end ANY theme preview. Like the plugin name suggests 🙂</li>
<li>Theme (zip on the fly) download (optional, of course )</li>
<li>Premium themes shopping cart</li>
</ul>

Front End Theme Preview

Allow users to preview and/or download/buy themes on the front end

System Fonts vs Front End Theme Preview