I've put this as a feedback/suggestion, in case there is something wrong with the editor and not me.
I tried using the editor to make some changes to my vantage premium child theme and the contact form 7 plugin. I'm a little bit of a newbie to CSS, but I tried to change a few things through the editor and nothing happened, except for the menu font size and color that I wanted to change.
I tried jetpack's custom editor and that seemed to do the job. I'm guessing the editor should do the same thing if I'm not wrong since you're editing the style.css file?