notifications-admin/app/assets/stylesheets/govuk_frontend_toolkit/stylesheets/_device-pixels.scss

@mixin device-pixel-ratio($ratio: 2) {
  @media only screen and (-webkit-min-device-pixel-ratio: $ratio),
  only screen and (min--moz-device-pixel-ratio: $ratio),
  only screen and (  -o-min-device-pixel-ratio: #{($ratio*10)}/10),
  only screen and (     min-device-pixel-ratio: $ratio),
  only screen and (             min-resolution: #{($ratio*96)}dpi),
  only screen and (             min-resolution: #{$ratio}dppx) { 
    @content;
  }
}