While Timothy's approach should work in theory and not be too difficult technically, there are issues with providing different stylesheets to different users. I ran across these while experimenting with user selectable color schemes.
The main problem is that not all of the 'style' of a Moodle site is contained in the CSS stylesheets. Some of the text and background colours are defined in the HTML using colors specified in config.php.
The second, slightly more philosophical problem, is that a teacher seeing a very different view from what the student sees could lead to problems e.g. using a text color that doesn't show up well against the student background color.