body { isolation: isolate; }
#page-particles {
  position: fixed;
  inset: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  pointer-events: none;
}
/* The fixed sky is the only section background. Even a translucent rectangular
   wash changes its black level at the section boundary and reveals a seam.
   Local light remains attached to the logo, phone and anatomy themselves. */
.has-page-particles .hero-bottom,
.has-page-particles .training-scene { background: transparent; }
@media print { #page-particles { display: none; } }