style.css

body,
html {
  height: 100%;
}

body {
  font-family: monospace;
  font-weight: bold;
  font-size: 20px;
  margin: 0;
  overflow: hidden;
  background-color: #222;
}

body,
#box {
  display: flex;
  align-items: center;
  justify-content: center;
}

#box {
  color: #aaa;
  height: 20em;
  width: 30em;
  max-width: 80%;
  max-height: 80%;
  background-color: #333;
  border-radius: 0.4em;
  border: 2px solid #555;
  position: relative;
  flex-direction: column;
  transition-duration: 0.2s;
  overflow: hidden;
}

#box:hover,
#box[drag] {
  border-color: #38f;
  color: #ddd;
}

#input {
  cursor: pointer;
  position: absolute;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  opacity: 0;
}

#display {
  width: 100%;
  height: 100%;
}

select,
button {
  padding: 0.6em 0.4em;
  margin: 0.5em;
  width: 15em;
  max-width: 100%;
  font-family: monospace;
  font-weight: bold;
  font-size: 16px;
  background-color: #444;
  color: #aaa;
  border-radius: 0.4em;
  border: 1px solid #555;
  cursor: pointer;
  transition-duration: 0.2s;
}

select:hover,
button:hover {
  background-color: #666;
  color: #ddd;
}