.scroll-top-button {
  position: fixed;
  right: clamp(18px, 3vw, 40px);
  bottom: clamp(18px, 3vw, 40px);
  z-index: 80;
  display: grid;
  place-items: center;
  width: 48px;
  height: 48px;
  padding: 0;
  border: 2px solid currentColor;
  border-radius: 50%;
  background: transparent;
  opacity: 0;
  visibility: hidden;
  pointer-events: none;
  transform: translateY(10px);
  transition: color .18s ease, opacity .18s ease, transform .18s ease, visibility .18s ease;
}

.scroll-top-button--visible {
  opacity: 1;
  visibility: visible;
  pointer-events: auto;
  transform: translateY(0);
}

.scroll-top-button--blue {
  color: #0077ad;
  filter: drop-shadow(0 1px 2px #ffffffcc);
}

.scroll-top-button--light {
  color: #fff;
  filter: drop-shadow(0 1px 3px #001827b3);
}

.scroll-top-button--visible:hover {
  transform: translateY(-4px);
}

@media (prefers-reduced-motion: reduce) {
  .scroll-top-button { transition: none; }
}