Just an example, but what I do is store my CSS in a media library, then just add:
to the head of my master template. This allows me to use my beloved Sublime Text for editing.
There is 1 slight issue, when you do a staging push sometimes the sheets don't travel up (maybe 1 out of 10 times). For this reason, whenever I do a push, I have to go in and manually trigger the media libraries to sync. All I use the media library for is for is 2 or 3 sheets so it only adds maybe 15 seconds to my work. The benefit is I never touch the built-in Kentico editor.
Gotta have my syntax highlighting!