I don't believe this is currently implemented at this time. However you can manipulate the category.tpl the following way:
Using the {cid} value, you can explicitly change the CSS based on the category number. Using a wrapper div or whatever you choose. I am currently manipulating div's to change styles and background image per category. This has worked for me. @psychobunny may implement a better solution for this but right now, this is the best way to do it and it works nicely.
<div class="category-{cid}> ... </div>
So the class would be .class-#, replace # with category number.
That's the easy part, the headache would be of course styling per category of course, but hey... :squirrel: it shouldn't be a problem either.