The facade of Daniel and Mirella Levinas’s mansion off Wisconsin Avenue looks like many white-brick houses in Georgetown. Greek revival details convey patrician heritage and conservative taste without a hint that behind the pilastered doorway is a pristine, ultra-modern museum. Inside unfolds a domestic variation on the gallery style known as “the white cube.” Walls and floors are rectilinear planes stripped of decorative flourishes, and rooms are a sea of white, relieved here and there by a patch of muted color on the sparse furnishings.