$(".stream-nav").on("click", function() {
    // A selector to match all cards in all streams
    var allStreamsCardsSelector = ".card";
    // A selector to match just this stream's cards
    // for this, we use the class with the name of the stream,
    // which is the same as this nav link's id and then the "-card" suffix.
    var thisStreamCardsSelector = "." + this.id + "-card";

    // First remove the highlight from all of the cards
    $(allStreamsCardsSelector).removeClass("card-highlight");
    // Then apply the highlight to just this stream's cards
    $(thisStreamCardsSelector).addClass("card-highlight");
});