As of right now you’d need to do this:
body #wpsp-53110 .wp-show-posts-image .wp-show-posts-image-overlay,
body #wpsp-53110 .wp-show-posts-image .wp-show-posts-image-overlay,
body #wpsp-53110 .wp-show-posts-image .wp-show-posts-image-overlay,
body #wpsp-53110 .wp-show-posts-image .wp-show-posts-image-overlay {
background-color: rgba(0,63,135,0.9) !important;
}You’d just need to change the 53110 to the proper list ID.