body, ::backdrop {
    background: #fff;
}

#fullscreenback-button {
    position: absolute;
    bottom: 5px;
    left: 5px;
    background: rgba(0,0,0,0.05);
    border: 0;
    width: 50px;
    height: 50px;
    border-radius: 50%;
    box-sizing: border-box;
    transition: transform .3s;
    cursor: pointer;
    display: flex;
    flex-direction: column;
    justify-content: center;
    z-index: 1000;
}

    #fullscreenback-button:hover {
        transform: scale(1.125);
    }