body{
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background-color: black;
  margin: 0;
}

.calculator {
  background-color: #333;
  border-radius: 10px;
  padding: 20px;
  box-shadow: 0 4px 8px rgba(0,0,0,0.3);
}

.display {
  margin-bottom: 15px;
}

#display {
  width: 100%;
  height: 60px;
  font-size: 24px;
  text-align: right;
  padding: 0 15px;
  background-color: black;
  color: white;
  border: 2px solid white;
  border-radius: 5px;
  box-sizing: border-box;
}

.btn {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-gap: 10px;
  max-width: 320px;
}

.zero {
  grid-column: span 2;
}

button {
  background-color:black;
  color: white;
  font-size: 20px;
  font-family: Arial, Helvetica, sans-serif;
  text-align: center;
  padding: 20px;
  border: 2px solid white;
  border-radius: 5px;
  cursor: pointer;
  transition: background-color 0.2s;
}

button:hover {
  background-color: #777;
}

button:active {
  background-color: #555;
}