:root{
  --btt-size:48px; --btt-radius:999px; --btt-shadow:0 6px 18px rgba(0,0,0,.18);
  --btt-bg:#111; --btt-bg-hover:#000; --btt-icon:#fff; --btt-offset:18px;
}
#back-to-top.btt{
  position:fixed; right:var(--btt-offset); bottom:var(--btt-offset);
  width:var(--btt-size); height:var(--btt-size); display:flex; align-items:center; justify-content:center;
  border:none; border-radius:var(--btt-radius); background:var(--btt-bg); color:var(--btt-icon);
  box-shadow:var(--btt-shadow); cursor:pointer; z-index:9999; opacity:0; visibility:hidden;
  transform:translateY(8px);
  transition:opacity .25s ease, transform .25s ease, visibility 0s linear .25s, background .2s ease;
}
#back-to-top.btt:hover{ background:var(--btt-bg-hover); }
#back-to-top.btt svg{ fill:currentColor; }
#back-to-top.btt.show{ opacity:1; visibility:visible; transform:translateY(0); transition:opacity .25s ease, transform .25s ease, visibility 0s; }
@media (prefers-reduced-motion: reduce){ html{ scroll-behavior:auto; } }
@media (max-width:480px){ :root{ --btt-size:56px; --btt-offset:14px; } }