// produces a partly opaque background using RGB alpha channel
// with a workaround for IE8 that doesn't support it
@mixin background-opacity(
    $background-color: null,
    $opacity: 1,
    $include-ie: $include-ie
) {
    $background-opaque: rgba($background-color, $opacity);
    
    background-image: none;
    background-color: $background-opaque;

    @if $include-ie {
        $ie-background-str: ie-hex-str($background-opaque);
        
        .#{$prefix}ie8 & {
            filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#{$ie-background-str}, endColorstr=#{$ie-background-str});
            zoom: 1;
        }
    }
}