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.

kostyatereshchuk

Admin Menu in Frontend adds an option “Show admin menu when viewing site” to your profile. You can enable this option and see the admin menu on the public pages.
Also, you can close the admin panel with returning to the recently viewed public page.
This solution may be good for content managers but not for frontend developers because admin menu may change the design of some themes.
On this video demonstration you can see how to show admin menu in frontend, hide the admin menu and close the admin panel.
<iframe loading="lazy" class="youtube-player" width="750" height="422" src="https://www.youtube.com/embed/3p6-w2sbkzc?version=3&rel=1&showsearch=0&showinfo=1&iv_load_policy=1&fs=1&hl=en-US&autohide=2&wmode=transparent" allowfullscreen="true" style="border:0;" sandbox="allow-scripts allow-same-origin allow-popups allow-presentation allow-popups-to-escape-sandbox"></iframe>
If you want only “Close admin panel” functionality, you can install <a href="https://wordpress.org/plugins/switcher/" rel="ugc">Switcher</a> plugin.

Admin Menu in Frontend

Works almost perfectly. The only issue I'm having are the sub menu's not showing up in some themes, due to z-index issues. Either the plugin needs to set a real high z-index value for the submenus, or have the option for the site admin to adjust that value.

Overall though, very good, so even with this little problem (easily fixable), I gladly give it 5 stars without any hesitations.

Follow up update:

Ok, I can't fix this with a simple z-index value. However, if I collapse the menu, the sub menus appear, which is a nice compromise I guess. So my guess is that this is a JavaScript issue? Again, the submenu issue happens with some themes, and not all. For example, this plugin works fine with the WP default themes.

Almost perfect.

Admin Menu in Frontend allows you to show admin menu when viewing site.

System Fonts vs Admin Menu in Frontend