.sudoku-grid {
  display: grid;
  grid-template-columns: repeat(9, 40px);
  grid-template-rows: repeat(9, 40px);
  gap: 1px;
  margin: 20px auto;
  width: fit-content;
}

.cell {
  border: 1px solid #ccc;
  text-align: center;
  font-size: 18px;
  line-height: 40px;
  cursor: pointer;
}

.cell[data-row="2"], .cell[data-row="5"] {
  border-bottom: 2px solid #000;
}
.cell[data-col="2"], .cell[data-col="5"] {
  border-right: 2px solid #000;
}

.cell.prefilled {
  background: #f0f0f0;
  font-weight: bold;
}
.cell.error {
  background: #ffcccc;
}