As every theme is different, it isn't really possible to provide an editing experience that reflects the individual styling of a theme, without that theme providing some sort of hint about how to do it. There have been some super interesting experiments in front-end editing that try to inherit some of the theme styles, to varying degrees of success. Unfortunately, that approach has tended to cause more problems than it solved.

That said, I think there are going to be some interesting areas to explore after WordPress 5.0, as the Gutenberg focus shifts to site customisation, and considering how the entire site can be built of blocks. That's were you'll see better integration between Custom CSS and post content.