Skinning in Flex 3 can be a bit of a nightmare as there is no code complete for CSS and it's often very hard to find documentation.
I ran into a small problem when wanting to create an Apple esq scroll bar for a data grid, firstly you have to apply scale-9 skinning to the thumb (the bit you drag) so it doesn't deform when the amount of data changes.
Then the main problem is that the Apple style arrows have kind of dock that the thumb slides into, which is a problem as Flex doesn't seem to let the thumb overlap the arrows, or offer you any height and width control for them.
So to work around this I use scale 9 on the track and added the start of the button (the bit that docks) to the top and bottom, as the thumb can slide over the track, giving the effect it's going into the button.
I expect that is rather confusing but take a look at the example...