/* 1. 回到頂部按鈕樣式 */
#scrollTopBtn {
    position: fixed; bottom: 100px; right: 20px;
    background: #000; color: #fff; padding: 10px 15px;
    border-radius: 5px; cursor: pointer; display: none;
    z-index: 9999; font-size: 12px; opacity: 0.7;
}