Custom CSS Outsourcer
Loads the additional Customizer CSS from external files instead of printing it directly to the page.
By default, any additional CSS written in the Customizer is printed directly to the head of the page. This tiny plugin changes it to load the CSS from a somewhat virtual file, using a custom rewrite rule, which can make sense to clean up your HTML code. The contents of the stylesheet will be cached.
This plugin naturally requires at least WordPress version 4.7 Beta since it is built on functionality introduced in that version.
It is recommended to use the stable version of the plugin which you can download at wordpress.org.
Of course you can also download the plugin from GitHub. Stable versions are available under Releases. If you're interested in a developmental version, you can download it here. Note that you should not use a development version in a production environment.