body {
  font-family: Arial, sans-serif;
  margin: 0;
  background: #121212;
  color: white;
  text-align: center;
}

header {
  padding: 15px;
  background: #1f1f1f;
}

.calculator {
  max-width: 320px;
  margin: 40px auto;
  background: #1f1f1f;
  padding: 20px;
  border-radius: 12px;
}

#display {
  width: 100%;
  padding: 20px;
  font-size: 32px;
  margin-bottom: 15px;
  text-align: right;
  border: none;
  border-radius: 10px;

  background: #1a1a1a;
  color: #ffffff;

  box-shadow: inset 0 2px 5px rgba(0,0,0,0.5);
  box-sizing: border-box;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

button {
  padding: 15px;
  font-size: 18px;
  border: none;
  border-radius: 8px;
  background: #333;
  color: white;
  cursor: pointer;
}

button:hover {
  background: #555;
}

.zero {
  grid-column: span 2;
}

footer {
  margin-top: 20px;
  font-size: 14px;
}