Some of j00 may remember an earlier version I made of this several years ago: a simulation of an amber monochrome display!

It sits on top of any existing light style - it looks best with Sakomoto IMHO. You can also change the hue-rotate value to somewhere between 50-80deg for a green variant

html{
  filter:
    contrast(0.85)
    invert(1)
    sepia(1)
    brightness(0.8)
    hue-rotate(-10deg) /*change to 50-80deg for green phospher*/
    saturate(5);
}

body {
  background-color: #ffffff;
  background-image: none;
  color: #666666;
}

iframe,
.theading,
.theading2,
.theading3,
img:not([src$=".svg"]) {
  filter:
    invert(1);
}

#usercounter {
  background-color: #ffffff;
  color: #000000
}