2.1.39 Part 4 Section 2.2.1, background (Document Background)
a. The standard states that any element from the VML namespace or the Office VML Drawing namespace is allowed as a child of the background element.
Word only allows a single instance of the VML background element to be a child of the background element.
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.