media query retina / no retina

media.css

@media only screen and (-webkit-min-device-pixel-ratio: 1) {
    &::before {
      background-image: url("./assets/bg.jpg");
    }
  }

  @media only screen and (-webkit-min-device-pixel-ratio: 2),
    only screen and (-o-min-device-pixel-ratio: 2/1),
    only screen and (min-resolution: 192dpi),
    only screen and (min-resolution: 2dppx) {
    &::before {
      background-image: url("./assets/bg@2x.jpg");
    }
  }