By default, Style is osStandard, meaning that the outline displays items in the style determined by the OutlineStyle property. Set Style to otOwnerDraw to create owner-draw outlines, meaning that items must be drawn on the canvas of the outline by code in an OnDrawItem event handler.  

Owner-draw outlines can display items other than the Text of a node and the standard bitmaps specified in the PictureClosed, PictureOpen, PictureMinus, PicturePlus, and PictureLeaf properties. Owner-draw outlines require more programming, however, as the application must specify how to render the image for each node in an OnDrawItem event handler. 

Canvas 

OnDrawItem 

OutlineStyle