.grain {
  position: fixed;
  inset: -100%;
  width: 300%;
  height: 300%;
  pointer-events: none;
  z-index: 9999;
  opacity: 0.035;
  background-image: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='300' height='300'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.75' numOctaves='4' stitchTiles='stitch'/%3E%3C/filter%3E%3Crect width='300' height='300' filter='url(%23n)'/%3E%3C/svg%3E");
  background-size: 200px 200px;
  background-repeat: repeat;
  animation: shift 0.5s steps(1) infinite;
}

@keyframes shift {
  0%   { transform: translate(0, 0); }
  20%  { transform: translate(-3%, 2%); }
  40%  { transform: translate(2%, -3%); }
  60%  { transform: translate(-1%, 4%); }
  80%  { transform: translate(3%, -1%); }
  100% { transform: translate(-2%, 3%); }
}