body {
  margin: 0;
  height: 100vh;
  max-height: 300vh;
  min-width: 580px;
  background-color: rgb(0, 0, 0);
}

headline {
  text-align: center;
}
headline h1 {
  color: rgb(0, 255, 65);
  font-family: "Courier New", Courier, monospace;
  font-size: 60px;
  text-shadow: 0px 0px 20px rgb(0, 255, 65);
}

calculator {
  display: flex;
  justify-content: center;
  margin-top: 50px;
}

#calculator {
  font-family: Arial, Helvetica, sans-serif;
  background-color: rgb(29, 29, 29);
  border-radius: 15px;
  max-width: 550px;
  min-width: 500px;
  overflow: hidden;
}

#display {
  width: 500px;
  padding: 20px;
  font-size: 5rem;
  text-align: left;
  background-color: rgb(59, 59, 59);
  color: rgb(255, 255, 255);
  border: none;
}

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

button {
  width: 100px;
  height: 100px;
  border-radius: 25px;
  border: none;
  background-color: rgb(0, 0, 0);
  color: rgb(255, 255, 255);
  font-size: 3rem;
  font-weight: bold;
  cursor: pointer;
  display: flex;
  justify-content: center;
  align-items: center;
  transition: 250ms ease-in-out;
}

button:hover {
  background-color: rgb(40, 40, 40);
  transform: scale(1.1);
}
button:active {
  background-color: rgb(82, 82, 82);
}

.operator {
  background-color: rgb(0, 255, 0);
}
.operator:hover {
  background-color: rgb(86, 255, 86);
}
.operator:active {
  background-color: rgb(135, 255, 135);
}

.side-op {
  background-color: rgb(60, 120, 60);
}
.side-op:hover {
  background-color: rgb(80, 120, 80);
}
.side-op:active {
  background-color: rgb(110, 120, 110);
}