ConstReadonlyCellBg: 3Cell background, drawn above both row layers.
ReadonlyNone: 0No background target.
ReadonlyRowBg0: 1Base row layer; replaces the automatic alternating row color.
ReadonlyRowBg1: 2Row overlay blended above RowBg0, useful for selection.
Background layers for TableSetBgColor(), drawn from RowBg0 to CellBg.