Environment Libraries allows themers to manage external css & js through the a Drupal admin UI & optionally with contexts (the recommended approach). Files support multiple versions based on prod/stage/dev environments for easier development on production instances. A basic role based permission allows access to non-production versions of a library.
This is a simple howto for moving single coded website to a multi-site environment. I had a few White Screens Of Death (WSOD) before I gained the wisdom to write this (and hopefully help others).