.xto-scrolltop {
    width: 40px;
    height: 40px;
    position: fixed;
    z-index: 1000;
    display: block;
    visibility: hidden;
    opacity: 0;
    background-position: center;
    background-repeat: no-repeat;
    transition: opacity .25s ease, visibility .25s ease;
}
.xto-scrolltop--dark { background-image: url('../img_dark.png'); }
.xto-scrolltop--light { background-image: url('../img_light.png'); }
.xto-scrolltop--visible { visibility: visible; opacity: var(--xto-scrolltop-opacity, .35); }
.xto-scrolltop:hover, .xto-scrolltop:focus { opacity: 1; }