#loadingBar {
    width: 100%;
    height: 3px;
    background-color: gray;
    animation-name: barra;
    animation-duration: 3s;
    animation-iteration-count:infinite;
   }
  
  @keyframes barra{
     0%   {background-color:gray;}
    50%  {background-color:black;}
    100% {background-color:gray;}
  }