I use a card-mod custom theme. Note this is out of date as I have not updated card-mod yet:
ha-card.top-level-card {
border: solid 1px var(--secondary-text-color);
background: linear-gradient(rgba(57, 103, 158, 1), rgba(0, 0, 0, 0.6) 56px, rgba(0, 0, 0, 0.6) 100%);