* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: 'Segoe UI', sans-serif;
}

body {
  height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;

  background: radial-gradient(circle at top, #0f172a, #020617);
  color: white;
}

.container {
  text-align: center;
}

.title {
  font-size: 42px;
  margin-bottom: 5px;
}

.subtitle {
  font-size: 18px;
  opacity: 0.8;
  margin-bottom: 10px;
}

.card {
  padding: 25px;
  border-radius: 20px;

  background: rgba(255,255,255,0.08);
  backdrop-filter: blur(18px);
  border: 1px solid rgba(255,255,255,0.15);

  box-shadow: 0 10px 40px rgba(0,0,0,0.5);
}

/* Sudoku board */
#sudoku-board {
  display: grid;
  grid-template-columns: repeat(9, 42px);
  grid-template-rows: repeat(9, 42px);
  justify-content: center;
  margin: 20px auto;
  gap: 2px;
}

/* Cells */
input {
  width: 42px;
  height: 42px;
  text-align: center;
  font-size: 18px;

  border: 1px solid rgba(255,255,255,0.2);
  outline: none;

  background: rgba(255,255,255,0.05);
  color: white;

  transition: 0.2s;
}

/* Focus effect */
input:focus {
  background: rgba(59,130,246,0.3);
  transform: scale(1.05);
}

/* Pre-filled numbers */
input.prefilled {
  background: rgba(255,255,255,0.2);
  font-weight: bold;
  color: #facc15;
}

/* Invalid cell */
input.invalid {
  background: rgba(239,68,68,0.4);
}

/* Button */
button {
  margin-top: 10px;
  padding: 12px 20px;
  border: none;
  border-radius: 12px;

  cursor: pointer;
  font-size: 16px;
  font-weight: bold;

  background: linear-gradient(135deg, #3b82f6, #8b5cf6);
  color: white;

  transition: 0.25s;
}

button:hover {
  transform: scale(1.05);
  background: linear-gradient(135deg, #22c55e, #3b82f6);
}

/* Message */
#message {
  margin-top: 15px;
  font-size: 18px;
}