* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}
body {
  background-color: #1e1e1e;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  font-family: 'Segoe UI', sans-serif;
}
.calculator {
  background: #2c2c2c;
  padding: 20px;
  border-radius: 15px;
  width: 320px;
  text-align: center;
  color: #fff;
}
#display {
  width: 100%;
  height: 50px;
  font-size: 24px;
  margin: 15px 0;
  padding: 10px;
  border: none;
  border-radius: 8px;
  background: #fefefe;
  color: #000;
  text-align: right;
}
.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 12px;
}
button {
  padding: 20px;
  font-size: 18px;
  background: #444;
  color: white;
  border: none;
  border-radius: 10px;
  cursor: pointer;
  transition: background 0.2s;
}
button:hover {
  background: #666;
}