html { scroll-behavior: smooth; }

  #toTopBtn {
    position: fixed;
    right: 16px;
    bottom: 16px;
    width: 44px;
    height: 44px;
    border: none;
    border-radius: 999px;
    box-shadow: 0 6px 20px rgba(0,0,0,.15);
    background: #757575;
    color: white;
    font-size: 18px;
    cursor: pointer;
    z-index: 1000;
    display: grid;
    place-items: center;
    opacity: 0;
    transform: translateY(8px);
    transition: opacity .2s ease, transform .2s ease;
  }

  #toTopBtn.show {
    opacity: 1;
    transform: translateY(0);
  }

  #toTopBtn:hover { filter: brightness(1.05); }
  #toTopBtn:active { transform: translateY(1px); }

  #toTopBtn:focus-visible {
    outline: 3px solid #fff;
    outline-offset: 2px;
    box-shadow: 0 0 0 4px rgba(31,111,235,.5);
  }