html {
  font-family: sans-serif;
  background-color: ivory;
}

div.center {
  text-align: center;
}

button {
  margin: 0.2em;
}

button.large {
  padding: 1em;
  width: 30em;
}

button.start {
  background-color: green;
  color: white;
}

button.stop {
  background-color: darkred;
  color: white;
}

div.header {
  position: sticky;
  top: 0;
  background-color: ivory;
  width: 100%;
}

@media screen and (max-width: 800px) {
  button.large {
    width: 100%;
  }
  h1 small {
    display: block;
  }
}