Upfront Feature Request: Custom CSS with responsive break points

I'm just looking at my site and creating some custom CSS and it occurred to me that it would be nice to have some way to easily specify the CSS the breakpoint.

You could add basic responsive media queries to the available element selectors, or a button to "add responsive CSS". Either way you'd want to allow the user to easily say:
This break point and smaller,
This break point and larger,
This break point only.