private void setCellStyleForMark(FocusWidget widget, Mark mark) {
    widget.removeStyleName(style.cellStateX());
    widget.removeStyleName(style.cellStateO());
    widget.removeStyleName(style.cellStateEmpty());

    switch (mark) {
      case X:
        widget.addStyleName(style.cellStateX());
        break;

      case O:
        widget.addStyleName(style.cellStateO());
        break;

      case EMPTY:
        widget.addStyleName(style.cellStateEmpty());
        break;
    }

    widget.setEnabled(mark == Mark.EMPTY);
  }