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.

Łukasz Nowicki

Post/Page/Product-specific custom code allows you to add custom CSS styles to individual posts, pages, or WooCommerce products. It provides a dedicated area in the editor screen where you can write your CSS code. You can also choose whether the CSS should apply only to the single view or also to archive-type views.
A new meta box will appear below the content editor on the edit screen for posts, pages, and products. You can enter any custom CSS there and decide whether it loads only on the single view or also on archive pages like category listings or product grids.
<h3>Requirements</h3>
This plugin requires at least WordPress 5.0 (remember always to keep your WordPress installation up to date!) and PHP 7.4 on your server.

Post/Page Specific Custom Code

Great plugin!

A very useful plugin

Add custom CSS to posts, pages, or WooCommerce products, with optional archive support. Includes a dedicated editor box.

Legacy Jetpack Custom CSS Editor vs Post/Page Specific Custom Code