@mixin retina($ratio: 1.5) {
$dpi: $ratio * 96;
$opera-ratio: $ratio * 100;
@media only screen and (-webkit-min-device-pixel-ratio: #{$ratio}),
only screen and ( -o-min-device-pixel-ratio: '#{$opera-ratio}/100'),
only screen and ( min-resolution: #{$dpi}dpi),
only screen and ( min-resolution: #{$ratio}dppx) {
@content;
}
}