Create a background area as defined by the given window. Only windows with »-background« receive a <rect/> representing their region. ttk-stuff won't be processed here.
highlightThickness isn't used here borderWidth: centered on the outer line, duplicate it to get the correct internal dimensions.
Windows are not independent from their container. If the container is a canvas window, then the canvas coordinate system must be used.
Raised and Sunken are rendered the same way. Groove will be dealt as a simple line, but with a lighter stroke, opposite to »solid«.