body {
background:#000;}	
@media screen and (max-width:800px){
  .principal {width:100%;height:525px;margin:auto;padding:10px;margin-top:20px;margin-bottom:50px;background:#000;text-align:center;min-height:610px;}
}

@media screen and (min-width:800.1px) and (max-width:1024px){
  .principal {width:690px;height:525px;margin:auto;padding:10px;margin-top:80px;margin-bottom:50px;background:#000;text-align:center;min-height:610px;}
}

@media screen and (min-width:1024.1px){
  .principal {width:690px;height:525px;margin:auto;padding:10px;margin-top:80px;margin-bottom:50px;background:#000;text-align:center;min-height:610px;}
}