Add CSS and JS snippets to your site, and activate them on demand. Instead of using the WordPress customizer, dealing with child themes or editing the actual theme files, this plugin makes it easy to add customizations to your sites. You can add as many snippets as needed, and deactivated them individually if debugging.
I didn’t classify this as a front-end plugin, as it doesn’t produce any output by itself, any validation and compliance of the code you add, is entirely up to you.
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Extract from WordPress.org
Customize your WordPress site’s appearance by easily adding custom CSS and JS code without even having to modify your theme or plugin files. This is perfect for adding custom CSS tweaks to your site.https://wordpress.org/plugins/custom-css-js/