Custom style is not applied to empty cells when applying to the entire row.
Workaround:
Dim usedCellRange As CellRange = mobjValidationResultWs.UsedCellRange
For rowIndex As Integer = usedCellRange.FromIndex.RowIndex To usedCellRange.ToIndex.RowIndex
For columnIndex As Integer = usedCellRange.FromIndex.ColumnIndex To usedCellRange.ToIndex.ColumnIndex
Dim cell As CellSelection = mobjValidationResultWs.Cells(rowIndex, columnIndex)
cell.SetStyleName("Common")
Next columnIndex
Next rowIndex