// Retina image
@mixin retina-image($file-1x, $file-2x, $width-1x, $height-1x){
    background-image: url("${file-1x}");

    @media
    only screen and (-webkit-min-device-pixel-ratio: 2),
    only screen and (   min--moz-device-pixel-ratio: 2),
    only screen and (     -o-min-device-pixel-ratio: 2/1),
    only screen and (        min-device-pixel-ratio: 2),
    only screen and (                min-resolution: 192dpi),
    only screen and (                min-resolution: 2dppx) {
        background-image: url("${file-2x}");
        background-size: $width-1x $height-1x;
    }
}