style.css

html,
body {
  min-height: 100%;
}
body {
  font-family: sans-serif;
  text-align: center;
  background: #111 url(https://goo.gl/D1AeUS) center no-repeat;
  background-size: cover;
  color: #aaa;
}

.container {
  display: flex;
  flex-direction: column;
  align-items: center;
}

.dosbox-container {
  margin: 10% auto 0 auto;
  width: 640px;
  height: 400px;
}

.dosbox-container > .dosbox-overlay {
  background: url(./assets/img/overlay.png) center no-repeat;
  background-size: 100%;
}

#dosbox:hover .dosbox-start {
  opacity: 1;
}

#dosbox .dosbox-start {
  background: url(https://static.tumblr.com/98vyz1x/eK4lj7luf/header.png) center no-repeat;
  background-size: 150px;
  height: 100%;
  image-rendering: pixelated;
  top: 4% !important;
  color: rgba(0, 0, 0, 0);
  filter: drop-shadow(0px 0px 8px rgba(255, 255, 255, 0.95));
  opacity: 0;
}

.dosbox-powered {
  opacity: 0;
}

button {
  border: 0;
  margin: 10px;
  cursor: pointer;
  font-weight: 700;
}

p {
  font-size: 32px;
}

a {
  color: #aaa;
}