It appears his full CSS is as follows:

  background-image: url(/css/noise.png);
  background-repeat: repeat;
  background-size: 182px;
  filter: brightness(6);
  opacity: 0.018;

In my experimentation with it, I can't quite figure out how he applies the effect to the whole page without it either dissapearing on adjustments to the viewport or affecting the pages text opacity.