* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: 'Arial', sans-serif;
}

body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background: #1e1e1e;
}

.calculator {
  background: #2d2d2d;
  padding: 20px;
  border-radius: 15px;
  box-shadow: 0 10px 25px rgba(0,0,0,0.5);
}

.display {
  background: #000;
  color: #0f0;
  font-size: 2rem;
  text-align: right;
  padding: 20px;
  border-radius: 10px;
  margin-bottom: 15px;
  overflow-x: auto;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 80px);
  grid-gap: 10px;
}

.btn {
  background: #444;
  color: #fff;
  font-size: 1.5rem;
  padding: 20px;
  border: none;
  border-radius: 10px;
  cursor: pointer;
  transition: 0.2s;
}

.btn:hover {
  background: #555;
}

.equal {
  background: #0f0;
  color: #000;
  grid-row: span 2;
}

.zero {
  grid-column: span 2;
}