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.

Need to add some custom CSS to your site? Simple CSS gives you an awesome admin editor and a live preview editor in the Customizer so you can easily add your CSS.
Want your CSS to only apply on a specific page or post? Simple CSS adds a metabox which allows you to do just that.
Check out GeneratePress, our awesome WordPress theme! (https://wordpress.org/themes/generatepress)
<h4>Features include:</h4>
<ul>
<li>Full featured admin CSS editor</li>
<li>Dark and light editor themes</li>
<li>CSS editor in the Customizer so you can live preview your changes</li>
<li>Metabox for page/post specific CSS</li>
</ul>

Simple CSS

Add CSS to your website through an admin editor, the Customizer or a metabox for page/post specific CSS.

Legacy Jetpack Custom CSS Editor vs Simple CSS