* {
  box-sizing: border-box;
  font-family: Arial, sans-serif;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  margin: 0;
  background: #f0f0f0;
}

.calculator {
  width: 300px;
  background: #ff4c4c; /* default red */
  padding: 20px;
  border-radius: 12px;
  box-shadow: 0 8px 20px rgba(0, 0, 0, 0.2);
  transition: background 0.3s ease;
}

.display {
  width: 100%;
  height: 50px;
  margin-bottom: 15px;
  text-align: right;
  font-size: 1.5rem;
  padding: 10px;
  border: none;
  border-radius: 8px;
  background: #fff;
}

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

button {
  padding: 15px;
  font-size: 1.2rem;
  border: none;
  border-radius: 8px;
  cursor: pointer;
  background: #fff;
  transition: 0.3s;
}

button:hover {
  background: #ddd;
}

.clear-btn {
  grid-column: span 4;
  background: #ff9999;
}

.toggle-btn {
  margin-top: 15px;
  width: 100%;
  padding: 12px;
  background: #333;
  color: #fff;
  font-size: 1rem;
  border-radius: 8px;
}