mdb-ui-kit/scss/includes/_dividers.scss

hr {
  &.on-dark {
    color: lighten($black, 10%);
  }

  &.on-light {
    color: lighten($white, 10%);
  }

  @media  (-webkit-min-device-pixel-ratio: 0.75),
    (min--moz-device-pixel-ratio: 0.75),
    (-o-device-pixel-ratio: 3/4),
    (min-device-pixel-ratio: 0.75),
    (min-resolution: 0.75dppx),
    (min-resolution: 120dpi) {
      height:0.75px;
    }

  @media  (-webkit-min-device-pixel-ratio: 1),
    (min--moz-device-pixel-ratio: 1),
    (-o-device-pixel-ratio: 1),
    (min-device-pixel-ratio: 1),
    (min-resolution: 1dppx),
    (min-resolution: 160dpi) {
      height:1px;
    }
  @media  (-webkit-min-device-pixel-ratio: 1.33),
    (min--moz-device-pixel-ratio: 1.33),
    (-o-device-pixel-ratio: 133/100),
    (min-device-pixel-ratio: 1.33),
    (min-resolution: 1.33dppx),
    (min-resolution: 213dpi) {
      height:1.333px;
    }
  @media  (-webkit-min-device-pixel-ratio: 1.5),
    (min--moz-device-pixel-ratio: 1.5),
    (-o-device-pixel-ratio: 3/2),
    (min-device-pixel-ratio: 1.5),
    (min-resolution: 1.5dppx),
    (min-resolution: 240dpi) {
      height:1.5px;
    }

  @media  (-webkit-min-device-pixel-ratio: 2),
    (min--moz-device-pixel-ratio: 2),
    (-o-device-pixel-ratio: 2/1),
    (min-device-pixel-ratio: 2),
    (min-resolution: 2dppx),
    (min-resolution: 380dpi) {
      height:2px;
    }

  @media  (-webkit-min-device-pixel-ratio: 3),
    (min--moz-device-pixel-ratio: 3),
    (-o-device-pixel-ratio: 3/1),
    (min-device-pixel-ratio: 3),
    (min-resolution: 3dppx),
    (min-resolution: 480dpi) {
      height:3px;
    }

  @media  (-webkit-min-device-pixel-ratio: 4),
    (min--moz-device-pixel-ratio: 4),
    (-o-device-pixel-ratio: 4/1),
    (min-device-pixel-ratio: 3),
    (min-resolution: 4dppx),
    (min-resolution: 640dpi) {
      height:4px;
    }

}