iframe.substack {position: fixed; top: 50%; left: 50%; z-index: 9999; transform: translate(-50%,-50%); display: none;}
iframe.substack + a {
  position: fixed;
  width: 100%;
  height: 100vh; 
  top: 0;
  left: 0;
  z-index: 9991;
  background: rgba(0,0,0,0.5);
  backdrop-filter: blur(5px);
  display: none;
}
iframe.substack.visible,
iframe.substack.visible + a {display: block;}
iframe.substack.visible.mobile,
iframe.substack.visible.mobile + a {display: none;}

@media only screen and (max-width: 600px) {
  iframe.substack.visible,
  iframe.substack.visible + a {display: none;}
  iframe.substack.visible.mobile,
  iframe.substack.visible.mobile + a {display: block;}
}