html, body {
    min-height: 100%;
    margin: 0;
 }

body { 
    min-height: 100%;
    padding: 1em;
    font: normal 1em / 1.5em  sans-serif;
 }
 
body::before {
    content: '';
    position: fixed;
    z-index: -1;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    background-image: linear-gradient( to bottom, #ccc, #444 );
 }
 
#person {
    position: fixed;
    z-index: 2;
    top: 50%;top: 25vw; 
    left: 0;
    width: 100%;
    padding: 0 0.5em;
    box-sizing: border-box;
    transform: translate( 0, -50% );transform: translate( 0, 0 );
    line-height: 1em;
    font-size: 2em;
    color: #fff; 
    text-align: center;
    animation: fadetext 9s forwards;
 }
 #person span {
    display: inline-block;line-height: 1em;
    font-size: 2em;
    margin: 0 2vw;
    transform: rotateY( 0deg ); 
    animation: rotatetext 9s forwards;
 }
 
@keyframes fadetext {
   0% { color: #fff; }
  80% { color: #fff; }
 100% { color: transparent; }
 }

 @keyframes rotatetext {
   0% { transform: rotateY( 0deg );}
  20% { transform: rotateY( 0deg );}
  80% { transform: rotateY( 360deg );}
 100% { transform: rotateY( 360deg );}
 }
 
body::after {
    content: ''; 
    position: absolute;
    z-index: 1;
    left: 0;
    right: 0;
    top: 0;
    bottom: 0;
    background-color: #000;
    animation: fadein 6s 2s forwards;
 }
 
@keyframes fadein {
 from { background-color: #000; z-index: 1; }
   to { background-color: transparent; z-index: -1; }
 }
 
label[for="draw-curtains"] {
    display: block;
    width: 10em;
    padding: 0.5em 0;
    border: 1px solid #999;
    border-radius: 0.5em;
    margin: auto;
    background-image: linear-gradient( to bottom, #fff, #999 );
    box-shadow: 0.3em 0.3em 0.3em rgba( 0, 0, 0, 0.4 );
    font-weight: bold;
    text-align: center;
    cursor: pointer;
 }

label[for="draw-curtains"]::before {
    content: 'Open ';
 }

#draw-curtains:checked ~ label[for="draw-curtains"]::before {
    content: 'Close ';
 }

#container {
    position: relative;
    width: 60%;
    padding-top: 33.75%;
    margin: 2em auto 0;
    border: 0.15em solid #000;
    box-shadow: 0 0 2em 1em rgba( 0, 0, 0, 0.5 );
    background-color: #fff;
    overflow: hidden;
 }

#birthday {
    position: absolute;
    top: 0;
    width: 100%;
    height: auto;
 }

#container div {
    position: absolute;
    top: 0;
    width: 50%;
    height: 100%;
    background-image: url(../images/curtains.jpg );
    background-size: cover;
    transition: left 3s ease-in-out;
 }

#container div:first-of-type {
    left: 0;
    box-shadow: 0 0 2em 0.75em rgba( 0, 0, 0, 0.6 );
 }

#container div:last-of-type {
    left: 50%;
    box-shadow: 0 -2em 2em 0.75em rgba( 0, 0, 0, 0.6 );
 }

#draw-curtains:checked ~ #container div:first-of-type {
    left: -50%;
 }

#draw-curtains:checked ~ #container div:last-of-type {
    left: 100%;
 }

@media ( max-width: 40em ) {
#container {
    width: 72%;
    padding-top: 40.5%;
  }
 }
 
@media ( max-width: 30em ) {
#container {
    width: 92%;
    padding-top: 51.75%;
  }
  
#person {
    font-size: 1em;
  } 
 }