body {
  font-family: Arial, sans-serif;
  background: linear-gradient(135deg, #4facfe, #00f2fe);
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
}

.calculator {
  background: #222;
  padding: 20px;
  border-radius: 15px;
  box-shadow: 0 5px 15px rgba(0,0,0,0.3);
  text-align: center;
  color: white;
  width: 300px;
}

h1 {
  margin-bottom: 15px;
}

#display {
  width: 100%;
  height: 50px;
  font-size: 22px;
  margin-bottom: 15px;
  text-align: right;
  padding: 10px;
  border-radius: 5px;
  border: none;
  color: #ffffff;
}

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

button {
  padding: 15px;
  font-size: 18px;
  border: none;
  border-radius: 8px;
  background: #444;
  color: white;
  cursor: pointer;
  transition: background 0.2s;
}

button:hover {
  background: #666;
}

button:active {
  background: #888;
}

button:nth-child(4n) {
  background: #ff9800; /* operator buttons */
}

button:nth-child(4n):hover {
  background: #e68900;
}

button:last-child {
  grid-column: span 2;
  background: #4caf50;
}

button:last-child:hover {
  background: #45a049;
}