WPF UserControl

Hi, I am trying to figure out how to keep the background of a UserControl always Null or transparent when a user of the control sets the background to a non-transparent brush.  Basically, let's say I have a border within a UserControl and I bind the background to the UserControl's background.  If I set the UserControl's background to Red, I want only the border's background to show up Red, but the remaining area in the UserControl not covered by the internal border should be transparent.  How can I do this?

Thanks,

Steve