Define it like this.
$retina_display: "(-webkit-min-device-pixel-ratio: 1.3), (-o-min-device-pixel-ratio: 2.6/2), (min--moz-device-pixel-ratio: 1.3), (min-device-pixel-ratio: 1.3)";
Use it like this.
#logo {
background: url(../img/logo.png) no-repeat 0 0;
width: 168px;
height: 67px;
@media #{$retina_display} {
background: url(../img/logo@2x.png) no-repeat 0 0;
background-size: 168px 67px;
}
}
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