style.css

* {
  box-sizing: border-box;
  margin: 0;
}

header {
  text-align: center;
  height: 100px;
}

h1 {
  line-height: 100px;
  font-family: 'Roboto', sans-serif;
  font-size: 3em;
}

td {
  border: 2px solid black;
  height: 180px;
  width: 180px;
  text-align: center;
  vertical-align: middle;
}

table {
  margin: 0 auto;
}

.cell {
  border-radius: 10px;
  font-family: 'Roboto', sans-serif;
  font-size: 3em;
}

#interface {
  height: 200px;
  width: 300px;
  margin: 0 auto;
  text-align: center;
}

#result {
  visibility: hidden;
  height: 70px;
  width: 90%;
  margin: 5px auto;
  text-align: center;
}

#result p {
  line-height: 70px;
  font-family: 'Roboto', sans-serif;
  font-size: 2em;
}

#button {
  height: 90px;
  width: 90%;
  background-color: transparent;
  border: black 2px solid;
  margin: 5px auto;
  font-family: 'Roboto', sans-serif;
  font-size: 2em;
}