deco should be the ID of a definition that is used to provide the graphics and metrics for the dialog frame. It should define in script:
FrameDecorationBorderTop, FrameDecorationBorderBottom, FrameDecorationBorderLeft, FrameDecorationBorderRight: Functions to return the border margins
FrameDecorationBackClr: RGBa value of solid background
The border graphics are defined as actions in the ActMap:
FrameDecoTopLeft, FrameDecoTopRight, FrameDecoBottomLeft, FrameDecoBottomRight: Corner elements
FrameDecoTop, FrameDecoBottom, FrameDecoLeft, FrameDecoRight: Bordeer elements
FrameDecoWall: Background element (not sure if that's actually implemented).