As a designer, there may be a certain look and feel we're going for that requires custom CSS. After all, the rich content editor is quite limited (try using a border radius or box shadow, for example). I think it would be easier to make style changes myself on the course level than to bother the Canvas admin about the global stylesheet.
In the settings page of the course, there should be a "Custom CSS" option where you can add new classes that can be called from anywhere in the course. This custom stylesheet could also be imported from one course to another so that transfers don't lose custom styling.