AFAIK, you're out of luck here. The styling for the scrollbar is defined independently from the rest of portal, and that styling is not currently exposed in the Inspector, so copying a portal's styling doesn't pick up the the scrollbar's styling. Presumably the reason for this is that the scrollbar is shared by multiple object types (portals and edit boxes), so it's defined in one place in the stylesheet rather separately for each of those object types.
Thanks Mark. I was afraid of that. FileMaker really needs to give us a way to control the appearance of scrollbars or give us a broader selection of presets to choose from.