for (index,cell) in enumerate(self.tableView.visibleCells){
            if let acell = cell as? ChatCell {
               acell.changeColor(self.isWhiteBackground)
            }
        }

      for (index,cell) in self.tableView.visibleCells.enumerate(){
            if let acell = cell as? ChatCell {
               acell.changeColor(self.isWhiteBackground)
            }
        }