I think I didn't describe accurately the problem. Style are working well, also when grouping (I've added a couple of attached behaviors and are working fine in both cases). The problem is only about row back colors. When no group is present, rows are taking background color from the custom RowStyle and Alternate Style. When grouping, Data Row background are not using the provided values in styles, but they are white for RowStyle and Orange (SelectionBackGround in OfficeBlack Theme) for AlternateRowStyle.