body {
  margin: 0;
  font-family: 'Segoe UI', sans-serif;
  background: #000;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

.calculator {
  width: 320px;
  padding: 20px;
}

.display {
  color: #fff;
  text-align: right;
  margin-bottom: 10px;
}

#expression {
  font-size: 1rem;
  color: #aaa;
  min-height: 20px;
}

#display {
  width: 100%;
  height: 60px;
  font-size: 2.5rem;
  background: none;
  border: none;
  color: #fff;
  text-align: right;
  padding: 10px 0;
}

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

button {
  height: 60px;
  border: none;
  border-radius: 50%;
  font-size: 1.5rem;
  color: white;
  background: #333;
  cursor: pointer;
  transition: background 0.2s;
}

button:hover {
  background: #444;
}

button.operator {
  background: #ff9500;
  color: white;
}

button.operator:hover {
  background: #e08900;
}

button.light {
  background: #a5a5a5;
  color: black;
}

button.zero {
  grid-column: span 2;
  border-radius: 30px;
  text-align: left;
  padding-left: 25px;
}