How to load different css file according to user.

:confused; Hi - on our high school network, I'd like to have a different
colored background on pages that are accessed by different user groups (eg)
primary and secondary students and staff. I don't want to duplicate the page. I
think the solution may lie in loading a different style sheet for each user
group. Could this be differentiated on some factor in the user's 'rights' or
'profile' (not sure of correct term)? Just a broad brush yes or no would be
sufficient at this stage (and a pointer to any helpful links!). I have a couple
of tutorials on loading css dynamically that I have to work through, but will
eventually need to tailor to this specific application. Many thanks Jon (in
Brunei)