body {
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background-color: hsl(0, 0%, 90%);
}
#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: right;
  border: none;
  background-color: hsl(0, 0%, 40%);
  color: white;
  box-sizing: border-box;
  overflow: hidden;
}
#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,
button:active {
  background-color: hsl(0, 0%, 50%);
}
.action-button {
  background-color: dodgerblue;
}
.action-button:hover {
  background-color: hsl(225, 73%, 57%);
}
.action-button.active,
.action-button:active {
  background-color: hsl(225, 73%, 67%);
}
.operator {
  background-color: forestgreen;
}
.operator:hover {
  background-color: hsl(147, 50%, 47%);
}
.operator.active,
.operator:active {
  background-color: hsl(147, 50%, 57%);
}
#delete {
  background-color: hsl(0, 0%, 50%);
}
#delete:hover {
  background-color: hsl(0, 0%, 60%);
}
#delete.active,
#delete:active {
  background-color: chocolate;
}