Add css file to the Admin Theme of the Commerce Kickstart distribution?
I would strongly suggest to create a subtheme for additional CSS. That's what subthemes are for, after all, and it's less work than to make a module. Simply use info file and it's stylesheets[all][] entry.

If you have time, you can investigate how AdaptiveTheme allows GUI editing of CSS, and implement this in your custom theme. AT is open-source after all, so it would be legal to copy-paste this functionality. It will make your life easier for future: