Specific category styles

I remember seeing somewhere a plugin that would allow you to apply different stylesheets for different categories/authors. But I dunno if that was for the single post view or not. What I’m looking for is a simple script (preferably php) that would enclose the post in a set of div tags if it belonged to a certain category (e.g. “reminders”). I’d like to do this so that the reminder posts don’t take up nearly as much space as a regular post. I already tried doing an if ($cat = 24) but that just applied it to every post because $cat is used at the top of the template to tell it what categories to show.