main.css

*, *::before, *::after { 
  box-sizing: border-box; 
}

header {
  margin-bottom: 50px;
}

.board-choices {
  margin: 20px 30px;
  height: 150px;
  width: 150px;
  line-height: 150px;
  font-size: 50px;
}

.control {
  width: 150px;
}

#notification {
  display: block;
}

#display {
  border: 1px solid black;
  border-radius: 5px;
  background-color: black;
  color: red;
  width: 100px;
  margin: 0 auto;
}

.switch {
  margin-top: 15px;
}

.switch strong {
  margin-right: 15px;
}