#backToTopBtn{position:fixed;bottom:15px;right:15px;width:45px;height:45px;color:#fff;border:none;border-radius:50%;display:flex;align-items:center;justify-content:center;font-size:18px;cursor:pointer;opacity:0;transition:opacity 0.3s ease-in-out;z-index:999}#backToTopBtn.show{opacity:1}