body {
  background: linear-gradient(120deg, #43cea2 0%, #185a9d 100%);
  font-family: 'Segoe UI', Arial, sans-serif;
  margin: 0;
  min-height: 100vh;
  text-align: center;
}
h1 {
  color: #185a9d;
  margin-top: 32px;
  margin-bottom: 12px;
  letter-spacing: 2px;
}
.sudoku-board {
  display: grid;
  grid-template-columns: repeat(9, 36px);
  grid-template-rows: repeat(9, 36px);
  gap: 2px;
  justify-content: center;
  margin: 0 auto 18px auto;
  background: #185a9d;
  border-radius: 8px;
  max-width: 340px;
}
.sudoku-cell {
  background: #fff;
  border: 1px solid #43cea2;
  font-size: 1.2em;
  text-align: center;
  width: 36px;
  height: 36px;
  outline: none;
  border-radius: 4px;
  transition: background 0.15s;
}
.sudoku-cell.prefilled {
  background: #eafaf1;
  color: #888;
  font-weight: bold;
}
.sudoku-cell.invalid {
  background: #ffd6d6;
}
.controls {
  margin-bottom: 24px;
}
button {
  background: #43cea2;
  color: #fff;
  border: none;
  border-radius: 8px;
  padding: 10px 28px;
  font-size: 1em;
  font-weight: 600;
  cursor: pointer;
  margin: 0 8px;
  transition: background 0.2s;
}
button:hover {
  background: #185a9d;
  color: #fff;
}
@media (max-width: 500px) {
  .sudoku-board { grid-template-columns: repeat(9, 9vw); grid-template-rows: repeat(9, 9vw); max-width: 98vw; }
  .sudoku-cell { width: 9vw; height: 9vw; font-size: 4vw; }
}