body {
  font-family: sans-serif;
  text-align: center;
  background-color: #f4f4f4;
}

#board {
  display: grid;
  grid-template-columns: repeat(3, 60px);
  grid-gap: 5px;
  justify-content: center;
  margin: 20px auto;
}

.cell {
  width: 60px;
  height: 60px;
  background-color: white;
  font-size: 2em;
  display: flex;
  justify-content: center;
  align-items: center;
  border: 1px solid #999;
  cursor: pointer;
}