html,
body {
  width: 100%;
  height: 100%;
  margin: 0;
  padding: 0;
  text-align: center;
  background: #333639;
  color: white;
  display: flex;
  flex-direction: column;
}
iframe {
  width: 100%;
  height: 100%;
  margin: auto;
  display: block;
}
button {
  background-color: green;
  width:50%;
  align-self: center;
  min-width: 390px;

  border: none;
  color: white;
  padding: 10px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 16px;
  margin: 4px 2px;
  cursor: pointer;
  border-radius: 5px;
}