Not sure what you guys have done with core, but I love the fact that I can now update custom CSS on a live site without the live site temporary reverting to the default CSS - and in turn needing the user to refresh before the new customised CSS takes effect.
Seamless.
Very nice.
To understand what I mean.
Open two windows.
One with the custom CSS editor and one with the live site.
Make changes to the CSS, save it and…
Voila, the live site updates in front of your eyes. No fuss.
The Discourse way!