html, body {
  height: 100%;
  width: 100%;
  overflow: hidden;
  margin:0;
  background-repeat:no-repeat;
  background-attachment:fixed;
  webkit-background-size: cover;
  -moz-background-size: cover;
  -o-background-size: cover;
  background-size: cover;
}
#start-button {
  position: absolute;
  top: 35%;
  left: 41.5%;
  background-color: gray;
  border: none;
  border-radius: 1vw;
  color: #fff;
  font-size: 2vw;
  font-weight: 600;
  padding: 1.2vw 3vw;
  cursor: pointer;
  user-select: none;
  font-family: 'Montserrat', sans-serif;
  z-index: 1;
  transition: all .2s ease-in-out;
}

#start-button:hover {
  color: gray;
  background-color: #fff;
}

#start-button:active {
  background-color: gray;
}