Use css to style a custom spark skin in Flex 4

http://seanhess.net/posts/flex_skins_and_styles


/** OverlaySkin.mxml**/
<s:Rect left="0" right="0" top="0" bottom="0">
        <s:fill>
            <s:SolidColor color="{getStyle('backgroundColor')}" alpha="{getStyle('backgroundAlpha')}"/>
        </s:fill>
</s:Rect>

/** styles.css **/
.overlay {
    background-color: #000000;
    background-alpha: 0.7;
    skinClass: ClassReference("skins.OverlaySkin");
}