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.

Access Keys

AccessPress Custom CSS is a FREE plugin to add your own custom css to your site that will override plugins and theme’s default stylesheets. With this plugin you can put custom css code without editing your theme and/or your plugins files (really useful in case of any theme/plugin update). This is very easy to use plugin.
<h4>Features:</h4>
<ul>
<li>Add your custom css to a site.</li>
<li>Syntax Highlighter supports(by CodeMirror) color and tab support for write a good css code.</li>
<li>Lightweight – Installs in seconds.</li>
<li>User friendly – very easy to configure and setup.</li>
<li>Super support – get support via email/support forum/online chat.</li>
</ul>
AccessPress Custom CSS works best with every WordPress theme. Its even more remarkable when used with popular themes like VMagazine and AccessPress Parallax.
<h4>Some Useful Links</h4>
<ul>
<li>Support Email: support@accesspressthemes.com</li>
<li>Support Forum Link: http://accesspressthemes.com/support/</li>
<li>Youtube channel link: https://www.youtube.com/watch?v=TjZNcVG3fDE</li>
<li>Facebook link: https://www.facebook.com/AccessPressThemes</li>
</ul>

Legacy Jetpack Custom CSS Editor vs AccessPress Custom CSS