[id] { scroll-margin-top: 100px; }  .back-to-top-btn { position: fixed; bottom: 4rem; right: 4rem; z-index: 1000; width: 200px; height: 50px; background: #d32f2f; color: white; border-radius: 10%; display: flex; align-items: center; justify-content: center; font-size: 2rem; font-weight: 500; text-decoration: none; box-shadow: 0 4px 12px rgba(0,0,0,0.3); transition: all 0.3s ease; cursor: pointer; opacity: 0; visibility: hidden; } .back-to-top-btn.show { opacity: 1; visibility: visible; } .back-to-top-btn:hover { background: #b71c1c; transform: translateY(-3px); }  @media (max-width: 2000px) { .hide-mobile { display: none; } }