// 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
) {
    @if $background-color == transparent {
        background-color: $background-color;
    } @else {
        $background-opaque: rgba($background-color, $opacity);

        background-image: none;
        background-color: $background-opaque;

        @if $include-ie {
            $ie-background-str: ie-hex-str($background-opaque);

            .#{$prefix}ie8 & {
                -ms-filter: "progid:DXImageTransform.Microsoft.gradient(startColorstr=#{$ie-background-str}, endColorstr=#{$ie-background-str})";
                zoom: 1;
            }
        }
    }
}