#slide,.print-slide{isolation:isolate}
#slide::before,.print-slide::before{
  content:"";position:absolute;inset:0;z-index:-2;pointer-events:none;
  background-image:var(--slide-bg,none);background-size:cover;background-position:center;
  opacity:var(--slide-bg-opacity,.3)
}
#slide::after,.print-slide::after{
  content:"";position:absolute;inset:0;z-index:-1;pointer-events:none;
  background:linear-gradient(90deg,rgba(11,13,16,.45),rgba(11,13,16,.16) 48%,rgba(11,13,16,.42)),
    linear-gradient(180deg,rgba(11,13,16,.05),rgba(11,13,16,.18));
  opacity:var(--slide-shade-opacity,1)
}

/* 逐页背景图待出图后按 README 素材 C.6 注册，例如：
#slide[data-slide-id="P17"],.print-slide[data-slide-id="P17"]{
  --slide-bg:url("assets/backgrounds/P17.webp");--slide-bg-opacity:.52}
*/

@media (prefers-reduced-transparency:reduce){
  #slide::before,.print-slide::before{opacity:.12}
}