body {
  background-color: #f9f9f9;
  padding-top: 40px;
}

.board {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(80px, 1fr));
  gap: 10px;
  max-width: 320px;
  margin: 0 auto;
}

.cell {
  aspect-ratio: 1 / 1;
  background-color: #fff;
  border: 2px solid #dee2e6;
  font-size: 2rem;
  line-height: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: background-color 0.2s;
  user-select: none;
}

.cell:hover {
  background-color: #e2f0ff;
}