2.1.34 Part 1 Section 17.2.1, background (Document Background)

a.   The standard states that any element from the VML namespace or the drawingML namespace is allowed as a child of the background element.

Word renders any drawingML specified as a background at the beginning of the document and not as a background.

b.   The standard does not state which setting is applied when both the themeShade and the themeTint attributes are present.

Word applies the themeTint setting when both of these attributes are present.