When "use CSS from theme" is selected it appears to be doing something like that. The problem is none of the themes look nice as email notifications. They weren't designed for that. You either end up with the really poor looking email notification you get for twenty eleven, or a nightmarish illegible mishmosh, depending on the theme. The blog themes just weren't designed for email notifications.
Compare with, say, the one-size-fits-all email notifications from Jetpack, which looks like the standard wordpress.com notifications (which ignore the current theme). At least those email notifications look very nice.
You would admit the idea of just "use the CSS from the current theme" results in bad looking notifications, right?
I'm just wondering if there are any samples of nice looking notification CSS that we can add to our current theme and then surround the email templates with appropriate divs so they make use of the added CSS. Even if it doesn't match the theme.