The editor should use the same code injection that is used on the front-end of your blog so that your custom code can be viewed closer to as it will appear on the website.
Sure, I can manually edit
/var/www/myblog.com/versions/2.1.1/core/built/assets/ghost.min-0fec1df6a718ea0ada6845c1df3119ff.css
and insert my code injection but it’s a bit of a pain to do that (I have to modify it to work with the editor), and I have to do it every time I update the front-end code injection or a new version is released. 
Anyways, just a thought… 