#back_to_top {
    cursor: pointer;
    position: fixed;
    right: -81px;
    bottom: 140px;
    z-index: 1000;
    width: 40px;
    height: 40px;
    background: url("//i.imgur.com/qAiD8nz.png") no-repeat top left/auto 100% transparent;
    transition: right .5s ease-in, background .3s ease-in-out;

    &.show-back-btn {
        right: 0;
    }

    &:hover {
        background-position: top right;
    }
}

@media @desktop {
    #back_to_top {
        width: 81px;
        height: 81px;
    }
}