body {
  margin: 0;
  min-height: 100vh;
  display: grid;
  place-items: center;
  background: #f2f2f2;
  font-family: Arial, sans-serif;
}

#calculator {
  width: 320px;
  background: #333333;
  border-radius: 22px;
  padding: 18px;
  box-shadow: 0 18px 40px rgba(0, 0, 0, 0.25);
}

#display {
  width: 100%;
  height: 95px;
  border: none;
  outline: none;
  border-radius: 16px;
  background: #333333;
  color: #ffffff;
  font-size: 2.6rem;
  text-align: right;
  padding: 0 16px 10px;
  box-sizing: border-box;
}

#keys {
  margin-top: 14px;
  background: #262626;
  border-radius: 18px;
  padding: 16px;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 14px;
  justify-items: center;
}

#keys button {
  width: 58px;
  height: 58px;
  border: none;
  border-radius: 50%;
  background: #4d4d4d;
  color: #ffffff;
  font-size: 1.6rem;
  font-weight: 600;
  cursor: pointer;
  user-select: none;
  transition: transform 0.06s ease, filter 0.06s ease;
}

#keys button:active {
  transform: scale(0.97);
  filter: brightness(1.12);
}