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.

SaifuMak

Add custom codes to your wordpress website. You can add custom PHP Functions, HTML, custom CSS, Javascript, Google Analytics, Search Console verification tags or other code snippets to your site.
Features
<ul>
<li>100% free wordpress plugin, no paid upgrades needed!</li>
<li>Add PHP Snippets, Custom CSS, Javascript, Google Analytics, Facebook Scripts, Meta Verification Codes, Search console verification and other codes to your wordpress website easily.</li>
<li>Generate Shortcodes for HTML Snippets and use it anywhere on the site!</li>
<li>Syntax highlighter enabled with Light and Dark Mode Editors</li>
<li>Classify snippets using tags</li>
<li>Add Notes to Snippets</li>
<li>Activate/Inactivate snippets with a single click</li>
<li>Auto-deactivate feature for PHP snippets that causes errors.</li>
<li>Add Custome Codes globally or on individual posts, pages, products and even on custom post types.</li>
<li>Option to select where to put snippets: before </head> or before </body> etc.</li>
<li>Import/Export Snippets to use on other sites.</li>
</ul>

Add Custom Codes – Insert Header, Footer, Custom PHP Snippets, CSS, Javascript

Plugin works faster. Contact support if any problem happens.

Works fast

Add custom codes to your wordpress site. A completely free plugin to add Custom PHP functions, HTML, CSS, Javascript, any other codes to your website.

Legacy Jetpack Custom CSS Editor vs Add Custom Codes – Insert Header, Footer, Custom PHP Snippets, CSS, Javascript