The easiest solution is to go to "Display Properties > Appearance tab > "Advanced" button > "Other advanced controls" button (this option will appear if WB is integrated into the dialog - if not, skip this step) > either click on window background in the preview, or select "Window" in the drop-down list > use the "Color 1" drop-down to select White color > click "OK", and then "Apply" - before exiting Display Properties dialog.

Note - this is only a temporary color change. The skin will revert back to the designed color when you reboot the machine, or reload WB.

To change the background colors permanently for a skin, you will need to use "SkinStudio" to edit the skin. This process is a bit more involved, though relatively painless once you know your way around the program. I will leave the explanation for this procedure to the far more competent users of SkinStudio - unless no one else speaks up, and you really want to use this method.