IT PAYS to dig deep into the themes and styles part of FileMaker 13. Rob Poelking shows a work around for a problem that was bugging him:
Have you tried setting the color or opacity of your portal object only to find that it doesn’t seem to take? That happened to me and it took looking at the CSS to realize that while my Portal object was set to clear, the portal row was set to opaque white. The portal background colors are controlled by 7 different styles: the portal ( normal, hover, pressed, in focus), the portal row (normal, alternate, active). Each of those can have a separate color fill and opacity.
Follow the link for the answer.