/* Bouton "retour en haut" – élégant + néon vert/lilas */
:root{
  --bt-bg: rgba(18,20,25,.85);
  --bt-border: rgba(255,255,255,.12);
  --bt-shadow: 0 8px 24px rgba(0,0,0,.25), 0 0 16px rgba(178,131,255,.25);
  --bt-shadow-hover: 0 10px 28px rgba(0,0,0,.35), 0 0 24px rgba(109,247,168,.45);
  --bt-accent-1: #6df7a8;   /* vert tendre */
  --bt-accent-2: #b283ff;   /* lilas */
}
@media (prefers-color-scheme: light){
  :root{
    --bt-bg: rgba(255,255,255,.88);
    --bt-border: rgba(0,0,0,.08);
    --bt-shadow: 0 8px 24px rgba(0,0,0,.12), 0 0 16px rgba(178,131,255,.20);
    --bt-shadow-hover: 0 10px 28px rgba(0,0,0,.18), 0 0 24px rgba(109,247,168,.40);
  }
}

#backToTop{
  position: fixed;
  right: 22px;
  bottom: 22px;
  z-index: 10000;
  width: 52px; height: 52px;
  border-radius: 999px;
  display: grid; place-items: center;
  background: var(--bt-bg);
  backdrop-filter: blur(6px) saturate(120%);
  -webkit-backdrop-filter: blur(6px) saturate(120%);
  border: 1px solid var(--bt-border);
  box-shadow: var(--bt-shadow);
  cursor: pointer;
  opacity: 0; transform: translateY(8px) scale(.98);
  pointer-events: none;
  transition: opacity .25s ease, transform .25s ease, box-shadow .25s ease;
}
#backToTop.show{
  opacity: 1; transform: translateY(0) scale(1);
  pointer-events: auto;
}
#backToTop svg{
  width: 22px; height: 22px;
  filter:
    drop-shadow(0 0 6px rgba(109,247,168,.65))
    drop-shadow(0 0 10px rgba(178,131,255,.45));
}
#backToTop:hover{ box-shadow: var(--bt-shadow-hover); }
#backToTop:focus-visible{
  outline: 2px solid var(--bt-accent-2);
  outline-offset: 3px;
}

/* Variante “pastille” animée autour de l’icône */
#backToTop::after{
  content: "";
  position: absolute; inset: 2px;
  border-radius: inherit;
  background: radial-gradient(90% 90% at 50% 50%,
              rgba(109,247,168,.15), rgba(178,131,255,.12) 45%, transparent 70%);
  pointer-events: none;
}