* {
  box-sizing: border-box;
}
body {
  font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
  background: linear-gradient(135deg, #1d1f21, #323437);
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
  margin: 0;
}
.calculator {
  background-color: #2c2f33;
  border-radius: 16px;
  box-shadow: 0 0 30px rgba(0, 0, 0, 0.3);
  padding: 20px;
  width: 320px;
}
.display {
  background-color: #1e1e1e;
  color: #fff;
  font-size: 2.2em;
  padding: 20px;
  border-radius: 10px;
  margin-bottom: 20px;
  text-align: right;
  overflow-x: auto;
}
.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 15px;
}
.button {
  background-color: #3b3f45;
  border: none;
  border-radius: 12px;
  padding: 20px;
  color: white;
  font-size: 1.3em;
  cursor: pointer;
  transition: background 0.3s;
}
.button:hover {
  background-color: #555b63;
}
.button.operator {
  background-color: #f39c12;
  color: #fff;
}
.button.operator:hover {
  background-color: #e67e22;
}
.button.equal {
  background-color: #27ae60;
  grid-column: span 2;
}
.button.equal:hover {
  background-color: #1e874b;
}
.button.clear {
  background-color: #e74c3c;
}
.button.clear:hover {
  background-color: #c0392b;
}