Retina/high-dpi media query mixin http://robots.thoughtbot.com/towards-a-retina-web-follow-up/

retina-mixin.scss

@mixin retina($ratio: 1.3) {
  @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-resolution: #{round($ratio*96)}dpi),
  only screen and (min-resolution: #{$ratio}dppx) {
    @content;
  }
}