#toolbar {
    background-color: #fff;
    transition: border-bottom 0.2s ease;
    box-shadow: rgba(0, 0, 0, .3) 0 0 4px;
    z-index: 1030;
}

/* 由 JS 加上去 */
#toolbar.is-fixed {
    position: fixed;
    top: 0;
    box-sizing: border-box;
}

#toolbar.stuck {
    border-bottom: 1px solid #ddd;
}