Scroller does not expose the displayed content in its skin

I want to add a border around the visible content of the scroller and of course that is not around the scrollbars. A custom Scroller skin seems to be cleaner way of doing this as this is the purpose of skins to change the visual rendering of a component.

Unfortunately, the default Scroller skin only exposes the scroll bars. There is no (at least easy) way to add this border.