Different CSS per page or category?

Does anybody know of a way that I can use a different CSS stylesheet depending on whether a post is in a specific category? Also, if there is a way to select a specific CSS stylesheet for a specif page (probably though selecting the page type)?

On my site at http://www.indie-film.tv, we have three film festivals, and I'd like to be able to use the festival's colour on posts/pages so that users can see exactly what it is meant for simply by the colours.