How to change past event thumbnail with CSS?

Site URL: https://www.fortsferryfarm.com/event-calendar

Since I disabled the day tag on the event page, the distinction between future and past event is not as clear as I would like, so I'd like to be able to use css to change the opacity of past event event thumbnails or overlay a semi-transparent white box over. If anyone can help moi out, it'd be much appreciated! 

Thank you 3000!

FYI template is brine

Add to Design > Custom CSS

/* Past event thumbnail overlay */
article.eventlist-event--past a.eventlist-column-thumbnail:after {
    content: "";
    background-color: rgba(0,0,0,0.75);
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 9999;
}
article.eventlist-event--past a.eventlist-column-thumbnail {
    position: relative;
}