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 …

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. This is particularly useful on multisite networks, where it’s often impractical to use child themes to save customizations for each site. Unfortunately, custom CSS is not visible to site admins on multisite networks by default because they are not trusted with unfiltered CSS.
This plugin gives site admins access to the core custom CSS feature by mapping the <code>edit_css</code> capability to <code>edit_theme_options</code>, thereby trusting site admins with unfiltered CSS.

Modular Custom CSS vs Multisite Custom CSS