GhostWire ScrollPane - Skinning
You generally would not be making skins for the ScrollPane.  The component reuses the ScrollBar component, which can be easily skinnned (read here).

After making your own scrollbar, you can simply replace the instance of the scrollbar used in the scrollpane component.  For an example of replacing the scrollbar, look into the ScrollPane(Silver) component.

Other than the scrollbar, if you are using the scrollpane with frame, you can customize the colors of the frame via the Properties Panel.  The colors you assign to Face, Highlight, Highlight3D, Shadow and DarkShadow will determine the look for the frame (it can be leveled flat or given the default bevel look).