body {
  font-family: sans-serif;
  text-align: center;
}
#sudoku-board {
  display: grid;
  grid-template-columns: repeat(9, 40px);
  grid-gap: 2px;
  justify-content: center;
  margin-top: 20px;
}
#sudoku-board input {
  width: 40px;
  height: 40px;
  text-align: center;
  font-size: 18px;
  border: 1px solid #999;
}
#sudoku-board input.prefilled {
  background-color: #eee;
  font-weight: bold;
}
.buttons {
  margin-top: 10px;
}
#message {
  margin-top: 10px;
  font-weight: bold;
}