index.css

body {
  display: flex;
}

#controls {
  margin-top: 30px;
}

.button {
  background-color: gray;
  border-radius: 15px;
  color: white;
  cursor: pointer;
  font-size: 2em;
  font-weight: bold;
  margin: 90px 40px;
  padding: 20px;
  text-align: center;
}

#traffic-light {
  background-color: #ababab;
  border-radius: 40px;
  margin: 30px 0;
  height: 550px;
  padding: 20px;
  width: 200px;
}

.bulb {
  background-color: #111;
  border-radius: 50%;
  margin: 25px auto;
  height: 150px;
  transition: background 300ms;
  width: 150px;
}

.stop {
  background-color: red;
}

.slow {
  background-color: orange;
}

.go {
  background-color: green;
}