body {
  background: linear-gradient(135deg, #74ebd5, #9face6);
  font-family: 'Poppins', sans-serif;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

.calculator {
  background: #222;
  padding: 20px;
  border-radius: 15px;
  box-shadow: 0 0 15px rgba(0,0,0,0.3);
  width: 260px;
}

#display {
  width: 100%;
  height: 50px;
  text-align: right;
  font-size: 22px;
  border: none;
  border-radius: 10px;
  padding: 10px;
  margin-bottom: 15px;
  background: #fff;
}

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

button {
  padding: 15px;
  font-size: 18px;
  border: none;
  border-radius: 10px;
  background: #333;
  color: white;
  cursor: pointer;
  transition: 0.2s;
}

button:hover {
  background: #555;
}

.equal {
  background: #2ecc71;
}

.equal:hover {
  background: #27ae60;
}

.zero {
  grid-column: span 2;
}