body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background: #111;
  font-family: Arial, sans-serif;
}

.calculator {
  background: #222;
  padding: 20px;
  border-radius: 15px;
  box-shadow: 0 5px 15px rgba(0,0,0,0.5);
}

#display {
  width: 100%;
  height: 60px;
  background: #000;
  color: #0f0;
  text-align: right;
  font-size: 2rem;
  border: none;
  border-radius: 10px;
  padding: 10px;
  margin-bottom: 15px;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 70px);
  gap: 10px;
}

.btn {
  height: 60px;
  background: #333;
  color: white;
  font-size: 1.2rem;
  border: none;
  border-radius: 10px;
  cursor: pointer;
  transition: 0.2s;
}

.btn:hover {
  background: #444;
}

.operator {
  background: #f39c12;
}

.equal {
  background: #27ae60;
  grid-column: span 2;
}