body {
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background: url(image/background.jpeg);
}
#calculator {
  font-family: Arial, sans-serif;
  background-color: hsl(0, 0%, 15%);
  border-radius: 15px;
  max-width: 500px;
  overflow: hidden;
}
#display {
  width: 100%;
  padding: 20px;
  font-size: 5rem;
  text-align: left;
  border: none;
  background-color: hsl(0, 0%, 20%);
  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: hsl(0, 0%, 30%);
  color: white;
  font-size: 3rem;
  font-weight: bold;
  cursor: pointer;
}

button:hover {
  background-color: hsl(0, 0%, 40%);
}
button:active {
  background-color: hsl(0, 0%, 50%);
}
.operator-btn {
  background-color: hsl(35, 100%, 55%);
}
.operator-btn:hover {
  background-color: hsl(35, 100%, 65%);
}
.operator-btn:active {
  background-color: hsl(35, 100%, 75%);
}
@media (max-width: 991px) {
  button {
    width: 70px;
    height: 70px;
  }
  #calculator {
    max-width: 350px;
  }
  #display {
    font-size: 3rem;
  }
}
@media (max-width: 500px) {
  button {
    width: 40px;
    height: 40px;
    font-size: 15px;
  }
  #calculator {
    max-width: 250px;
  }
}