.site-header{
  position:sticky;top:0;z-index:1000;
  background:rgba(255,255,255,.9);
  backdrop-filter:saturate(140%) blur(14px);
  border-bottom:1px solid transparent;
  transition:box-shadow .2s ease,border-color .2s ease,background .2s ease;
}
.site-header.is-scrolled{
  border-bottom-color:var(--line,#dfece5);
  box-shadow:0 10px 30px rgba(16,48,45,.08);
}
.site-header .nav{height:76px}
.back-to-top{
  position:fixed;right:22px;bottom:22px;z-index:950;
  width:46px;height:46px;border:1px solid rgba(16,48,45,.12);
  border-radius:50%;background:#10302d;color:#fff;
  display:grid;place-items:center;font-size:22px;line-height:1;
  cursor:pointer;box-shadow:0 14px 34px rgba(16,48,45,.2);
  opacity:0;transform:translateY(12px) scale(.96);pointer-events:none;
  transition:opacity .2s ease,transform .2s ease,background .2s ease;
}
.back-to-top:hover{background:#087e60;transform:translateY(-2px) scale(1)}
.back-to-top.is-visible{opacity:1;transform:translateY(0) scale(1);pointer-events:auto}
@media(max-width:620px){
  .site-header .nav{height:68px}
  .back-to-top{right:16px;bottom:16px;width:44px;height:44px}
}
@media(prefers-reduced-motion:reduce){
  .site-header,.back-to-top{transition:none}
  .back-to-top:hover{transform:none}
}