Growl notifications can be styled using css.
Has anyone made any inroads on styling the notifications in Notification Center in Mountain Lion?
Notification center, like most Apple standard features doesn't provide much in the way of customizable appearance. Even app developers don't have any control over the appearance of the notifications. The most you can do is choose between the Banner and Alert styles in Notificantion preferences:
If you need more styling options, you'll have to keep using Growl where you can.
I found this little app called NCbackgrounder here:
I think this is what you were looking for. It works pretty well for me so far.