Complex sentence: It is both a major stylistic advance in Christus' oeuvre and that of Netherlandish portraiture, not just that the sitter is no longer set against a flat, neutral background; here she is placed in an airy, three-dimensional, realistic setting.
Split into simpler sentences: It is both a major stylistic advance in Christus' oeuvre and that of Netherlandish portraiture. The sitter is no longer set against a flat, neutral background, but placed in an airy, three-dimensional, realistic setting.