John Regan

Add Custom CSS to your WordPress site without any hassles.
An easy-to-use WordPress Plugin to add custom CSS styles that override Plugin and Theme default styles. This plugin is designed to meet the needs of administrators who would like to add their own CSS to their WordPress website. Styles created with this plugin will render even if the theme is changed.
New in Version 4.0.7
<ul>
<li>Tested for compatibility with WP 6.7.2</li>
<li>Tested for compatibility with PHP 8.4</li>
</ul>
<h3>Features</h3>
<ul>
<li>AMP Support</li>
<li>Customizer Control (live preview)</li>
<li>Useful Code Syntax Highlighter</li>
<li>Code linting (error checking)</li>
<li>No configuration needed</li>
<li>Simple interface built on native WordPress UI</li>
<li>Virtually no impact on site performance</li>
<li>No complicated database queries</li>
<li>Thorough documentation</li>
<li>Allows Administrator access on WP Networks (Multisite)</li>
</ul>
<h3>Usage</h3>
<ol>
<li>
Navigate to Appearance > Custom CSS in the Admin Menu
</li>
<li>
Enter in valid CSS styles
</li>
<li>
Click “Update Custom CSS”
</li>
<li>
View your changes in the Front End of your website
</li>
</ol>
<h3>Help</h3>
<a href="https://github.com/johnregan3/simple-custom-css/wiki" title="Simple Custom CSS Wiki" rel="nofollow ugc">Simple Custom CSS Wiki</a>
<a href="https://wordpress.org/support/plugin/simple-custom-css" title="Support Forum" rel="ugc">Support Forum</a>

Simple Custom CSS Plugin

This is one of the best Custom CSS Plugin I've ever used. I've been using it since 2016. This plugin works very well.

Simple but works really good.

Add Custom CSS to your WordPress site without any hassles.

benohead

amazingweb GmbH

With this plugin, you can easily add custom CSS styles to override styles provided in themes and plugins without having to modify their code. In this way your modifications will remain after an update of the theme or plugins.
You define CSS styles in a nice editor with syntax highlighting, undo/redo, find/replace, zoom in/out and line wrapping. 
If the editor is too small, you can switch to full screen mode and switch back when you are done.
Before being applied, your code will be compressed (minified) in order to make sure your site still loads as fast as possible.
If you do not want to apply the defined CSS styles everywhere on your site, there are many possibilities to suppress those styles:
<ul>
<li>on specific post or page types</li>
<li>on specific posts or pages</li>
<li>for logged in users</li>
<li>for specific post formats</li>
<li>for specific languages (requires qTranslate or mqTranslate to be installed)</li>
<li>for specific URL paths</li>
<li>for specific referrers</li>
<li>for specific IP addresses</li>
</ul>
Simple to use, powerful and with a low footprint. Just give it a try !

WP Style It

Add Custom CSS to your WordPress site without modifying your theme or plugins.

Simple Custom CSS Plugin vs WP Style It