.mother-box {
  border: 5px solid black;
  height: 474px;
  width: 472px;
}

.row:nth-child(3) {
  border-bottom: 3px solid black;
}

.row:nth-child(6) {
  border-bottom: 3px solid black;
}

.row {
  display: flex;
  flex-direction: row;
}

.cell {
  display: flex;
  justify-content: center;
  align-items: center;
  border: 1px solid black;
  height: 48px;
  width: 48px;
}

.cell:nth-child(9n - 6) {
  border-right: 3px solid black;
}

.cell:nth-child(9n - 3) {
  border-right: 3px solid black;
}