$retina: "only screen and (-webkit-min-device-pixel-ratio: 2), only screen and (min--moz-device-pixel-ratio: 2), only screen and (-o-min-device-pixel-ratio: 2/1), only screen and (min-device-pixel-ratio: 2), only screen and (min-resolution: 192dpi), only screen and (min-resolution: 2dppx)";
@mixin retina-background($filename, $extension, $bgsize: cover) {
background-image: url("#{$filename}.#{$extension}");
@media #{$retina} {
background-image: url("#{$filename}@2x.#{$extension}");
-webkit-background-size: $bgsize;
background-size: $bgsize;
}
}