body {
  background: #121212;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  font-family: 'Arial', sans-serif;
  margin: 0;
}

.calculator {
  width: 90%;
  max-width: 400px;
  background: #1f1f1f;
  padding: 20px;
  border-radius: 20px;
  box-shadow: 0 10px 25px rgba(0,0,0,0.5);
}

.display {
  background: #0d0d0d;
  color: #58a6ff;
  font-size: 3rem;
  text-align: right;
  padding: 25px;
  border-radius: 15px;
  margin-bottom: 20px;
  word-wrap: break-word;
  min-height: 70px;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 15px;
}

.btn {
  padding: 20px;
  font-size: 1.8rem;
  border: none;
  border-radius: 15px;
  background: #2c2c2c;
  color: #fff;
  cursor: pointer;
  transition: 0.2s;
}

.btn:hover {
  background: #3a3a3a;
}

.btn.operator {
  background: #444444;
  color: #ffcc00;
}

.btn.equal {
  background: #58a6ff;
  color: #fff;
  grid-row: span 2;
}

.btn.clear {
  background: #ff4c4c;
}

.btn.zero {
  grid-column: span 2;
}

@media (max-width: 480px) {
  .display {
    font-size: 2.5rem;
    padding: 20px;
  }
  .btn {
    padding: 18px;
    font-size: 1.5rem;
  }
}