* {
   margin: 0;
   padding: 0;
}

@-webkit-keyframes pulse {
   0% {background-color: #000000;}
   100% {background-color: #2B2B2B;}
}

body {
   -webkit-animation: pulse 5s infinite alternate;
   background-color: #000000;
}

.main {
   position: absolute;
   top: 50%;
   left: 50%;
   width: 256px;
   height: 256px;
   margin-top: -128px;
   margin-left: -128px;
}