.bingo-cell {
    position: relative;
    border: 2px solid #ccc;
    text-align: center;
    cursor: pointer;
}

.bingo-cell > .overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: limegreen;
    opacity: 0.35;
    z-index: 1;
}

#game-over-overlay {
    height: 100%;
    width: 100%;
    position: fixed;
    z-index: 1;
    top: 0;
    left: 0;
    background-color: rgb(0,0,0);
    background-color: rgba(0,0,0, 0.9);
    overflow-x: hidden;
}

#game-over-overlay .close-btn {
    position: absolute;
    top: 1em;
    right: 1em;
    font-size: 2em;

    text-decoration: none;
    color: #818181;
    display: block;
    transition: 0.3s;
}

#game-over-overlay .close-btn:hover, #game-over-overlay .close-btn:focus {
    color: #f1f1f1;
}