body {
  text-align: center;
  font-family: Arial, sans-serif;
  background-color: gray;
}
.led {
  width: 100px;
  height: 100px;
  border-radius: 50%;
  background-color: red;
  margin: 20px auto;
  box-shadow: 0 0 40px rgba(255, 0, 0, 1);
  transition: background-color 0.3s, box-shadow 0.3s;
}