Create a Panel that covers the window (its invisible). Then use SetPanelColor or SetPanelImage
http://www.blitzbasic.com/bpdocs/command.php?name=SetPanelColor&ref=goto
http://www.blitzbasic.com/bpdocs/command.php?name=SetPanelImage&ref=goto
http://www.blitzbasic.com/bpdocs/command.php?name=SetPanelColor&ref=goto
http://www.blitzbasic.com/bpdocs/command.php?name=SetPanelImage&ref=goto