When you use theme’s Post offer system, you can find that it has expiration function. To make it works, you can fill expiration end date in Post offer section. When a post becomes expired, it will have the label “expired”. You can add additional styling and make the whole post as grey-scale color. For this, add next:
.rh-expired-class, .expired_coupon{filter: grayscale(100%);-webkit-filter: grayscale(100%);}
To make black ‘Buy’ button, add this:
.rh-expired-class .btn_offer_block, .expired_coupon .btn_offer_block{background-color:black !important}
to Theme option – General – Custom CSS.