.promo_box_wrapper.promo_box_right .desc_wrapper a.button { background: #000 !important; color: #FFF !important; }
Comments
the buttons are the same around whole site and we do not have any option to change them individually per items/shortcodes. So if you want to change it's color for promo box only, you need to use custom css:
.promo_box_wrapper.promo_box_right .desc_wrapper a.button { background: #000 !important; color: #FFF !important; }