media-query.scss

@mixin mq($keyword) {
    /* … */
    @if $keyword == retina {
        @media 
            only screen and (-webkit-min-device-pixel-ratio: 1.3)
            only screen and (min-resolution: 124.8dpi)
            only screen and (min-resolution: 1.3dppx) {
                @content;
        }
    }
}