comprehensive-rust/theme/css/redbox.css

div#aspect-ratio-helper {
  position: fixed;
  top: 8px;
  left: 8px;
  right: 8px;
  z-index: 1000;
  pointer-events: none;
}

div#aspect-ratio-helper div {
  position: relative;
  outline: 2em solid rgba(255, 0, 0, 0.2);
  margin: 0 auto;
  /* At this width, the theme fonts are readable in a 16
       person conference room. If the browser is wider, the
       text becomes too small to be legible. */
  max-width: 980px;
  /* On a standard 16/9 monitor, we expect to lose a bit
       of vertical space to borders. */
  aspect-ratio: 16/8.5;
}

#turn-off-red-box {
  position: absolute;
  bottom: 10px;
  right: 10px;
  z-index: 10000;
  padding: 10px;
  background-color: #f44336;
  color: white;
  border: none;
  border-radius: 5px;
  cursor: pointer;
  pointer-events: auto;
}