body::before {
  content: " ";
  display: block;
  position: fixed;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  background: linear-gradient(rgb(0 0 0 / 30%) 50%, rgb(0 0 0 / 0%) 50%);
  z-index: 2;
  background-size: 100% 2px, 3px 100%;
  pointer-events: none;
}

pro tip: you can view other people's CSS code by right clicking on the page, clicking "view page source", and using ctrl+F to search for custom_css!