*{padding: 0px; margin: 0px;}

#centro{width: 100%; height: 100%; background: #333; margin: 0 auto;}

#topo {width: 100%; height: 20%; background: #f00; text-align: center;}
#topo h1{color: #fff; font-size: 2em;}
#padding-colunas{padding: 10px;}
.colunas2{width: 50%; float: left;background: #999; text-align: center; height: 30%;}

@media screen and (min-width: 320px) and (max-width: 568px)
{
  #centro {width: 100%;}
  #topo {background: #FF0;}
  #topo h1{color: #f00;}
  .colunas2{width: 100%;float: none; background: #f00; margin-top: 5px;}
  }