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 edit_css capability to edit_theme_options, thereby trusting site admins with unfiltered CSS.