body {
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background-color: #ffffffd8;
}

#calculator {
  font-family: Arial, Helvetica, sans-serif;
  background-color: #2d2e30;
  border-radius: 15px;
  max-width: 500px;
  overflow: hidden;
}

#display {
  width: 100%;
  padding: 20px;
  font-size: 5rem;
  text-align: left;
  border: none;
  background-color: #414244;
  color: white;
}

#keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
  padding: 25px;
}

button {
  width: 100px;
  height: 100px;
  border-radius: 50px;
  border: none;
  background-color: rgba(0, 0, 0, 0.623);
  color: white;
  font-size: 3rem;
  font-weight: bold;
  cursor: pointer;
}

button:hover {
  background-color: rgba(61, 60, 60, 0.8);
}

button:active {
  background-color: rgba(107, 103, 103, 0.8);
}

.operator-btn {
  background-color: rgb(175, 118, 12);
}

.operator-btn:hover {
  background-color: rgba(255, 195, 83, 0.8);
}

.operator-btn:active {
  background-color: rgba(255, 242, 61, 0.8);
}

/* Mobile (max-width: 480px) */
@media (max-width: 480px) {
  #calculator {
    width: 90%;
    max-width: 320px;
  }

  #display {
    font-size: 2.5rem;
    padding: 15px;
  }

  button {
    width: 60px;
    height: 60px;
    font-size: 2rem;
  }
}

/* Tablet (min-width: 481px) and (max-width: 768px) */
@media (min-width: 481px) and (max-width: 768px) {
  #calculator {
    width: 90%;
    max-width: 400px;
  }

  #display {
    font-size: 3.5rem;
    padding: 18px;
  }

  button {
    width: 80px;
    height: 80px;
    font-size: 2.5rem;
  }
}

/* Desktop (min-width: 769px) */
@media (min-width: 769px) {
  #calculator {
    width: auto;
    max-width: 500px;
  }

  #display {
    font-size: 5rem;
    padding: 20px;
  }

  button {
    width: 100px;
    height: 100px;
    font-size: 3rem;
  }
}