I'm just tossing this out since I haven't tested it, but I suspect that you could place an image control first to create a background, then place your functional controls on top of that. Clumsy, but possibly effective.
AFAIK there is no direct way to change to card color or background.
That's what I was hoping not to do. I was hoping there was something that I could use to "hack" the actual card file that would allow me to apply a background.
I've been told images on the cards requires the system to ping epdm for the image every time you display a data card or cycle down a list of items. This might pose performance issues and unnecessarily increase network traffic.