George Stephanis

This plugin re-adds a standalone editing page for Custom CSS 
in the WordPress Admin UI, for users that aren’t comfortable 
using the one in the Customizer.
Currently it only works with Jetpack and Jetpack’s added features, 
but shortly it will work without Jetpack as well, providing a 
more basic text editor without syntax highlighting.

Legacy Jetpack Custom CSS Editor

I was utterly dismayed to see the old Jetpack CSS editor disappear in favor of a little customizer box in WP 4.7. Thank you so much for restoring the legacy editor where we can actually see what we're working with, and where our designer can work with CSS without having to upload style.css every time they make a change. It's a vital part of our workflow and the little box in the customizer is completely inadequate.

many thanks for this lifesaver

This plugin re-adds the full page admin Custom CSS editor to Jetpack.

Theme Junkie

This plugin will enable a <a href="https://www.theme-junkie.com/plugins/theme-junkie-custom-css/" rel="nofollow ugc">custom css manager</a> on administration page to add Custom CSS code to your WordPress website. It will automatically override any theme or plugin default styles. It also very useful if you want to add customization to your website but do not want to edit your theme or plugin css files.
It comes with two ways to add the custom css code:
1. Setting 
You can go to Appearance → Custom CSS, then you will see a big box/textarea. Put your css code there.
2. Customizer – Live Preview 
If you want to see the live preview while you adding the custom css code, then you can go to Appearance → Customize, after that open the Custom CSS section tab.
<h4>Features Include:</h4>
<ul>
<li>No configuration needed</li>
<li>Live preview</li>
<li>Easy-to-use</li>
<li>Child theme alternative to add customization</li>
<li>Uninstall procedure</li>
</ul>
<h4>Plugin Info</h4>
<ul>
<li>Developed by <a href="https://www.theme-junkie.com/?utm_source=wporg&utm_medium=text_link&utm_campaign=Site%20Promotion" rel="nofollow ugc">Theme Junkie</a></li>
<li>Check out the <a href="https://github.com/themejunkie/theme-junkie-custom-css" rel="nofollow ugc">Github</a> repo to contribute.</li>
</ul>

TJ Custom CSS

Simply works

Easily to add any Custom CSS code to your WordPress website.

Legacy Jetpack Custom CSS Editor vs TJ Custom CSS