Nick Halsey

WordPress core provides custom CSS functionality in the customizer that’s specific to the current theme; you can switch themes freely with each theme’s additional CSS remaining in place. Sometimes, you need some CSS to apply regardless of the current theme persistently. This plugin adds a plugin CSS option for CSS that’s global and persists across theme changes.
With the Customizer, your CSS is instantly live-previewed, offering the ability to see exactly how your site will look before you publish your changes. The plugin CSS option is stored as an <code>option</code>. Prior to WordPress 4.7 (which introduced additional CSS in core), theme-specific CSS was stored as a <code>theme_mod</code>, in 4.7 and newer this is migrated to the core CSS functionality (which is theme-specific).

Modular Custom CSS

WordPress core provides custom CSS functionality in the customizer that's specific to the current theme; you can switch themes freely with each t …

Andrew Norcross

Reaktiv

The Reaktiv CSS Builder allows site owners to add custom CSS for any part of the site directly in the dashboard. Unlike other CSS builders, this plugin creates an actual CSS file and loads that instead of putting it all in the head, allowing the file to be cached.
Features
<ul>
<li>CSS editor contains live syntax highlighting</li>
<li>Multisite compatible</li>
<li>Import and export capabilities</li>
</ul>

Reaktiv CSS Builder

Add user generated custom CSS directly in the dashboard.

Modular Custom CSS vs Reaktiv CSS Builder