body {
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background-color: rgb(216, 216, 216);
}

#calculator {
  font-family: Arial, sans-serif;
  background-color: rgb(54, 54, 54);
  border-radius: 15px;
  max-width: 500px;
  width: 100%;
  margin: 10px;
  overflow: hidden;
}

#display {
  height: 4rem;
  width: 100%;
  padding: 15px;
  font-size: 3rem;
  text-align: left;
  border: none;
  background-color: rgb(69, 69, 69);
  color: white;
}

#keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
  padding: 20px;
}

button {
  width: 80%;
  aspect-ratio: 1 / 1; /* makes buttons stay square */
  border-radius: 50%;
  border: none;
  background-color: hsl(0, 0%, 30%);
  color: white;
  font-size: 2rem;
  font-weight: bold;
  cursor: pointer;
}

button:hover {
  background-color: hsl(0, 0%, 40%);
}
button:active {
  background-color: hsl(0, 0%, 50%);
}

.operater-btn {
  background-color: hsl(39, 100%, 50%);
}
.operater-btn:hover {
  background-color: hsl(39, 100%, 65%);
}
.operater-btn:active {
  background-color: hsl(39, 100%, 75%);
}

/* 📱 Mobile responsive */
@media (max-width: 600px) {
  #display {
    font-size: 2rem;
    padding: 10px;
  }

  button {
    font-size: 1.5rem;
  }

  #keys {
    gap: 8px;
    padding: 15px;
  }
}