.lightning{
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-image: url("../img/thunder.png");
  background-position: center center;
  background-origin: content-box;
  background-size: cover;
  background-attachment: fixed;
  -webkit-filter: brightness(3);
  filter: brightness(3);
  -o-filter: brightness(3);
  -moz-filter: brightness(3);
  z-index: -1;
}

/*Now just a opacity animation*/
.flashit{
-webkit-animation: flash ease-out 7s infinite;
-moz-animation: flash ease-out 7s infinite;
animation: flash ease-out 7s infinite;
animation-delay: 2s;
}

@-webkit-keyframes flash {
	from { opacity: 0; } 
  92% { opacity: 0; }
	93% { opacity: 0.6; }
  94% { opacity: 0.2; }
  96% { opacity: 0.9; } 
	to { opacity: 0; }
}

@keyframes flash {
	from { opacity: 0; } 
    92% { opacity: 0; }
	93% { opacity: 0.6; }
    94% { opacity: 0.2; }
    96% { opacity: 1; } 
	to { opacity: 0; }
}