The website and cms use separate css and images, so you can style them separately. You can find the style sheets and images inside the /css and /images folders respecitvely.
To change the website banner, just copy your PNG image over (cms folder)/images/website/banner.PNG. (note that the .PNG must be in capitals). To change the cms banner, just copy your PNG image over (cms folder)/images/cms/banner.PNG (note that the .PNG must be in capitals)