Please look at the attached picture. You see a form which uses a normal skin (AlterMetro)where I changed the border definition to show some transpareny. At the corners the transparent pixels are working but not on the sides (I assume that is the part that stretches). It keeps showing the purple pixels which should also be transparent. I think this maybe a bug.
FYI, I'm using the latest version 12.19, Delphi 2007 and Windows 7