.cell {
  width: 40px;
  height: 40px;
  text-align: center;
  font-size: 24px;
  font-weight: bold;
  border: 1px solid black;
  background-color: #ffffff;
}

.cell.initial {
  background-color: #eeeeee;
}

.cell.error {
  background-color: #ffdddd;
}

.block {
  border: 2px solid black;
}