/* Examine the Past — lightweight SVG subject atmospheres */
body.lesson-subject-bg{
  --lesson-theme:url("themes/world-history.svg");
  position:relative;
  background:
    radial-gradient(circle at 18% 0%,rgba(148,255,83,.035),transparent 26rem),
    radial-gradient(circle at 82% 6%,rgba(255,242,0,.03),transparent 24rem),
    #060806;
  isolation:isolate;
}
body.lesson-subject-bg::before{
  content:"";
  position:fixed;
  z-index:-3;
  inset:72px 0 0;
  background-image:var(--lesson-theme);
  background-size:cover;
  background-position:center center;
  background-repeat:no-repeat;
  opacity:.9;
  pointer-events:none;
  transform:translateZ(0);
  animation:etpAmbientDrift 28s ease-in-out infinite alternate;
}
body.lesson-subject-bg::after{
  content:"";
  position:fixed;
  z-index:-2;
  inset:72px 0 0;
  background:
    linear-gradient(90deg,
      rgba(2,5,2,.16) 0%,
      rgba(2,5,2,.30) 16%,
      rgba(3,7,3,.78) 34%,
      rgba(3,7,3,.92) 50%,
      rgba(3,7,3,.78) 66%,
      rgba(2,5,2,.30) 84%,
      rgba(2,5,2,.16) 100%);
  pointer-events:none;
}
body.subject-us-history{--lesson-theme:url("themes/us-history.svg")}
body.subject-world-history{--lesson-theme:url("themes/world-history.svg")}
body.subject-black-history{--lesson-theme:url("themes/black-history.svg")}
body.subject-world-religions{--lesson-theme:url("themes/world-religions.svg")}
body.subject-geography{--lesson-theme:url("themes/geography.svg")}
body.subject-civics-government{--lesson-theme:url("themes/civics-government.svg")}

body.lesson-subject-bg main.wrap{position:relative;z-index:1}
body.lesson-subject-bg .hero{
  background:linear-gradient(180deg,rgba(7,9,7,.20),rgba(7,9,7,.05));
  border-radius:0 0 22px 22px;
}
body.lesson-subject-bg .block,
body.lesson-subject-bg .collection-card,
body.lesson-subject-bg .featured-lesson,
body.lesson-subject-bg .hub-note{
  backdrop-filter:blur(2px);
  -webkit-backdrop-filter:blur(2px);
}
body.lesson-subject-bg .block{
  background:linear-gradient(150deg,rgba(23,29,23,.92),rgba(10,14,10,.95));
}
body.lesson-subject-bg .collection-card{
  background:linear-gradient(145deg,rgba(23,26,23,.91),rgba(9,13,9,.95));
}
body.lesson-subject-bg .sidebar .toc{
  background:rgba(7,10,7,.90);
  backdrop-filter:blur(3px);
}
#etpSiteHeader,#etpSiteFooter{position:relative;z-index:20}
#etpSiteHeader{position:sticky}

@keyframes etpAmbientDrift{
  0%{transform:translate3d(0,0,0) scale(1)}
  100%{transform:translate3d(0,-10px,0) scale(1.012)}
}
@media(max-width:900px){
  body.lesson-subject-bg::before{inset:72px 0 0;background-size:auto 100%;opacity:.65}
  body.lesson-subject-bg::after{inset:72px 0 0;background:rgba(3,6,3,.74)}
}
@media(max-width:560px){
  body.lesson-subject-bg::before{inset:62px 0 0;opacity:.46}
  body.lesson-subject-bg::after{inset:62px 0 0;background:rgba(3,6,3,.80)}
}
@media(prefers-reduced-motion:reduce){
  body.lesson-subject-bg::before{animation:none}
}