body {
  background: #000000;
  font-family: 'San Francisco', Arial, sans-serif;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}
.iphone-calculator {
  background: #222;
  border-radius: 40px;
  box-shadow: 0 8px 32px rgba(0,0,0,0.3);
  width: 320px;
  padding: 24px 16px 32px 16px;
  display: flex;
  flex-direction: column;
  align-items: center;
}
.display {
  background: #111;
  color: #fff;
  font-size: 1.2em;
  border-radius: 16px;
  width: 100%;
  min-height: 60px;
  margin-bottom: 16px;
  padding: 16px;
  text-align: right;
  box-sizing: border-box;
  letter-spacing: 1px;
  overflow-y: auto;
}
.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 12px;
  width: 100%;
}
.btn {
  background: #333;
  color: #fff;
  border: none;
  border-radius: 50%;
  font-size: 1.3em;
  padding: 18px 0;
  cursor: pointer;
  transition: background 0.2s;
}
.btn:active {
  background: #444;
}
.btn.orange {
  background: #ff9500;
  color: #fff;
}
.btn.gray {
  background: #a5a5a5;
  color: #222;
}
.btn.zero {
  grid-column: span 2;
  border-radius: 32px;
  text-align: left;
  padding-left: 32px;
}