:root{
  --ink:#0a1727;
  --navy:#082d5f;
  --navy-deep:#031a39;
  --blue:#176fca;
  --blue-2:#4ca8e8;
  --sky:#e7f6ff;
  --paper:#ffffff;
  --soft:#f4f8fc;
  --line:#cbdceb;
  --muted:#52677d;
  --gold:#f2aa19;
  --green:#167e69;
  --shadow:0 18px 48px rgba(8,45,95,.13);
  --radius:20px;
  --font-body:"Inter","Segoe UI",Arial,sans-serif;
  --font-display:"Barlow Condensed","Arial Narrow",Arial,sans-serif;
}
*{box-sizing:border-box}
html{scroll-behavior:smooth;background:var(--soft);color:var(--ink);font-family:var(--font-body)}
body{min-width:320px;margin:0;background:linear-gradient(180deg,#dff2ff 0,#f5f9fd 21rem,#f5f9fd 100%)}
button,input{font:inherit}
a{color:inherit}
.skip{position:fixed;left:-9999px;top:.75rem;z-index:100;padding:.75rem 1rem;border-radius:10px;background:#fff;color:var(--navy);font-weight:800}
.skip:focus{left:.75rem}
.topbar{position:sticky;top:0;z-index:30;border-bottom:1px solid rgba(255,255,255,.2);background:rgba(3,26,57,.96);color:#fff;box-shadow:0 8px 30px rgba(3,26,57,.18);backdrop-filter:blur(14px)}
.topbar-inner{display:grid;width:min(1180px,calc(100% - 2rem));min-height:68px;margin:auto;grid-template-columns:auto minmax(0,1fr) auto;align-items:center;gap:1rem}
.back-link{display:inline-flex;align-items:center;gap:.5rem;color:#dff2ff;font-size:.84rem;font-weight:800;text-decoration:none}
.back-link:hover,.back-link:focus-visible{text-decoration:underline;text-underline-offset:4px}
.book-title{min-width:0;text-align:center}
.book-title strong{display:block;overflow:hidden;color:#fff;font-family:var(--font-display);font-size:1.25rem;font-weight:800;letter-spacing:.035em;text-overflow:ellipsis;text-transform:uppercase;white-space:nowrap}
.book-title span{display:block;color:#91d8ff;font-size:.7rem;font-weight:800;letter-spacing:.12em;text-transform:uppercase}
.index-toggle{display:none;align-items:center;gap:.45rem;padding:.55rem .7rem;border:1px solid rgba(255,255,255,.28);border-radius:10px;background:rgba(255,255,255,.08);color:#fff;font-size:.78rem;font-weight:800;cursor:pointer}
.reader-shell{display:grid;width:min(1180px,calc(100% - 2rem));margin:1.35rem auto 5rem;grid-template-columns:310px minmax(0,1fr);align-items:start;gap:1.35rem}
.reader-index{position:sticky;top:86px;max-height:calc(100vh - 105px);overflow:auto;border:1px solid var(--line);border-radius:var(--radius);background:rgba(255,255,255,.94);box-shadow:var(--shadow)}
.index-head{position:sticky;top:0;z-index:2;padding:1.1rem 1.05rem .85rem;border-bottom:1px solid var(--line);background:rgba(255,255,255,.97)}
.index-kicker{margin:0 0 .25rem;color:var(--blue);font-size:.7rem;font-weight:900;letter-spacing:.13em;text-transform:uppercase}
.index-head h2{margin:0;font-family:var(--font-display);font-size:1.75rem;line-height:1;text-transform:uppercase}
.index-search{width:100%;margin-top:.8rem;padding:.7rem .8rem;border:1px solid #b8cbdd;border-radius:10px;background:#f8fbfe;color:var(--ink);font-size:.9rem}
.index-search:focus{outline:3px solid rgba(23,111,202,.2);border-color:var(--blue)}
.concept-list{display:grid;gap:.25rem;margin:0;padding:.6rem;list-style:none}
.concept-link{display:grid;width:100%;grid-template-columns:2rem minmax(0,1fr);align-items:start;gap:.55rem;padding:.65rem .6rem;border:0;border-radius:11px;background:transparent;color:var(--ink);text-align:left;cursor:pointer}
.concept-link:hover,.concept-link:focus-visible{background:var(--sky);outline:none}
.concept-link.is-active{background:var(--navy);color:#fff}
.concept-link.is-hidden{display:none}
.concept-number{display:grid;width:1.8rem;height:1.8rem;place-items:center;border-radius:50%;background:#dcecf8;color:var(--navy);font-size:.7rem;font-weight:900}
.concept-link.is-active .concept-number{background:var(--gold);color:#241700}
.concept-copy strong{display:block;font-size:.84rem;line-height:1.22}
.concept-copy small{display:block;margin-top:.2rem;color:var(--muted);font-size:.68rem;line-height:1.25}
.concept-link.is-active small{color:#cce8fa}
.reader-main{min-width:0}
.progress-card{display:grid;grid-template-columns:auto minmax(0,1fr) auto;align-items:center;gap:.8rem;margin-bottom:.85rem;padding:.8rem 1rem;border:1px solid var(--line);border-radius:15px;background:rgba(255,255,255,.88);box-shadow:0 8px 24px rgba(8,45,95,.07)}
.progress-card span,.progress-card strong{font-size:.78rem}
.progress-card span{color:var(--muted);font-weight:700}
.progress-card strong{color:var(--navy)}
.progress-track{height:7px;overflow:hidden;border-radius:999px;background:#dce8f2}
.progress-track i{display:block;width:0;height:100%;border-radius:inherit;background:linear-gradient(90deg,var(--blue),var(--blue-2));transition:width .25s ease}
.loading-card,.error-card{padding:2rem;border:1px solid var(--line);border-radius:var(--radius);background:#fff;box-shadow:var(--shadow)}
.error-card{border-left:5px solid #b42318}
.concept-article{overflow:hidden;border:1px solid var(--line);border-radius:var(--radius);background:var(--paper);box-shadow:var(--shadow)}
.concept-hero{position:relative;padding:clamp(1.35rem,4vw,2.6rem);overflow:hidden;background:linear-gradient(132deg,var(--navy-deep),var(--navy) 64%,#135f9d);color:#fff}
.concept-hero:after{position:absolute;right:-5rem;bottom:-6.5rem;width:15rem;height:15rem;border:1.3rem solid rgba(255,255,255,.07);border-radius:50%;content:""}
.concept-step{position:relative;z-index:1;display:inline-flex;margin-bottom:1.2rem;padding:.35rem .6rem;border:1px solid rgba(255,255,255,.24);border-radius:999px;background:rgba(255,255,255,.08);color:#bde9ff;font-size:.72rem;font-weight:900;letter-spacing:.1em;text-transform:uppercase}
.concept-hero h1{position:relative;z-index:1;max-width:18ch;margin:0;font-family:var(--font-display);font-size:clamp(2.7rem,7vw,5.4rem);font-weight:900;line-height:.88;letter-spacing:-.025em;text-transform:uppercase}
.concept-theme{position:relative;z-index:1;margin:.9rem 0 0;color:#96ddff;font-size:.78rem;font-weight:900;letter-spacing:.1em;text-transform:uppercase}
.concept-summary{position:relative;z-index:1;max-width:760px;margin:1.2rem 0 0;color:#eff9ff;font-size:clamp(1rem,2vw,1.18rem);line-height:1.65}
.concept-body{padding:clamp(1.25rem,4vw,2.7rem)}
.reading-copy{font-size:1.03rem;line-height:1.78}
.reading-copy h2{margin:2.2rem 0 .75rem;color:var(--navy);font-family:var(--font-display);font-size:2rem;line-height:1;text-transform:uppercase}
.reading-copy h2:first-child{margin-top:0}
.reading-copy h3{margin:1.8rem 0 .65rem;color:var(--navy);font-size:1.2rem}
.reading-copy p{margin:.8rem 0}
.reading-copy strong{color:#071e3c}
.reading-copy code{padding:.12rem .3rem;border-radius:5px;background:#eaf2f8;color:#0b4b83;font-size:.9em}
.reading-copy ul,.reading-copy ol{margin:.8rem 0;padding-left:1.4rem}
.reading-copy li{margin:.42rem 0;padding-left:.18rem}
.reading-copy hr{height:1px;margin:2rem 0;border:0;background:var(--line)}
.table-wrap{max-width:100%;margin:1rem 0;overflow-x:auto;border:1px solid var(--line);border-radius:12px}
.reading-copy table{width:100%;border-collapse:collapse;font-size:.9rem}
.reading-copy th,.reading-copy td{min-width:8rem;padding:.7rem .8rem;border-bottom:1px solid var(--line);vertical-align:top;text-align:left}
.reading-copy th{background:#e8f4fc;color:var(--navy);font-weight:900}
.reading-copy tr:last-child td{border-bottom:0}
.study-box{margin-top:1.5rem;padding:1.1rem 1.15rem;border:1px solid var(--line);border-radius:15px;background:#f7fbfe}
.study-box h2{display:flex;align-items:center;gap:.55rem;margin:0 0 .75rem;color:var(--navy);font-family:var(--font-display);font-size:1.5rem;text-transform:uppercase}
.study-box ul{margin:0;padding-left:1.2rem}
.study-box li{margin:.55rem 0;line-height:1.55}
.mnemonic-grid{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:.75rem}
.mnemonic{padding:.9rem;border-left:4px solid var(--gold);border-radius:8px;background:#fff7e4}
.mnemonic strong{display:block;color:#563700;font-size:1rem}
.mnemonic span{display:block;margin-top:.35rem;color:#664f25;font-size:.86rem;line-height:1.45}
.reference-details{margin-top:.75rem;border:1px solid var(--line);border-radius:12px;background:#fff}
.reference-details summary{padding:.85rem 1rem;color:var(--navy);font-weight:900;cursor:pointer}
.reference-details div{padding:0 1rem 1rem;color:#334a61;font-size:.9rem;line-height:1.6}
.reference-details ul{margin:0;padding-left:1.2rem}
.related-links{display:flex;flex-wrap:wrap;gap:.5rem}
.related-links a{padding:.48rem .68rem;border:1px solid #a9c8df;border-radius:999px;background:#fff;color:#0b568f;font-size:.78rem;font-weight:800;text-decoration:none}
.related-links a:hover,.related-links a:focus-visible{border-color:var(--blue);background:var(--sky)}
.concept-actions{display:grid;margin-top:1.6rem;padding-top:1.2rem;border-top:1px solid var(--line);grid-template-columns:minmax(0,1fr) minmax(0,1fr);gap:.75rem}
.action-primary,.action-secondary{display:flex;min-height:52px;align-items:center;justify-content:center;padding:.75rem 1rem;border-radius:12px;font-weight:900;text-align:center;text-decoration:none;cursor:pointer}
.action-primary{border:1px solid var(--navy);background:var(--navy);color:#fff;box-shadow:0 8px 18px rgba(8,45,95,.18)}
.action-primary:hover,.action-primary:focus-visible{background:#104b8e}
.action-primary.is-disabled{border-color:#9fb1bf;background:#d9e2e9;color:#526371;box-shadow:none;cursor:not-allowed}
.action-secondary{border:1px solid #aac7dd;background:#eaf6fe;color:var(--navy)}
.action-secondary:hover,.action-secondary:focus-visible{border-color:var(--blue);background:#d9effd}
.question-note{grid-column:1/-1;margin:0;color:var(--muted);font-size:.76rem;text-align:center}
.mobile-actions{display:none}
.hub-body{background:linear-gradient(180deg,#072e61 0,#0f5b9e 23rem,#edf6fc 23rem,#f5f9fd 100%)}
.hub-top{width:min(1160px,calc(100% - 2rem));margin:auto;padding:1.1rem 0;color:#fff}
.hub-brand{display:flex;align-items:center;justify-content:space-between;gap:1rem}
.hub-brand a{font-weight:900;text-decoration:none}
.hub-brand nav{display:flex;gap:1rem;font-size:.8rem;font-weight:800}
.hub-hero{width:min(1160px,calc(100% - 2rem));margin:2.6rem auto 2rem;color:#fff}
.hub-kicker{margin:0 0 .75rem;color:#91dcff;font-size:.76rem;font-weight:900;letter-spacing:.16em;text-transform:uppercase}
.hub-hero h1{max-width:12ch;margin:0;font-family:var(--font-display);font-size:clamp(3.8rem,9vw,7.8rem);font-weight:900;line-height:.78;letter-spacing:-.035em;text-transform:uppercase}
.hub-hero h1 span{color:#8dd9ff}
.hub-lead{max-width:650px;margin:1.5rem 0 0;color:#e8f7ff;font-size:1.05rem;line-height:1.65}
.hub-stats{display:flex;flex-wrap:wrap;gap:.6rem;margin-top:1.1rem}
.hub-stats span{padding:.45rem .7rem;border:1px solid rgba(255,255,255,.25);border-radius:999px;background:rgba(255,255,255,.08);font-size:.76rem;font-weight:800}
.hub-main{width:min(1160px,calc(100% - 2rem));margin:0 auto 4rem}
.subject-grid{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:1rem}
.subject-card{position:relative;display:flex;min-height:260px;overflow:hidden;flex-direction:column;padding:1.35rem;border:1px solid #bad3e6;border-radius:22px;background:#fff;box-shadow:var(--shadow);text-decoration:none;transition:transform .18s ease,box-shadow .18s ease}
.subject-card:hover,.subject-card:focus-visible{transform:translateY(-4px);box-shadow:0 24px 60px rgba(8,45,95,.19);outline:3px solid rgba(76,168,232,.28);outline-offset:3px}
.subject-card:after{position:absolute;right:-1.8rem;bottom:-2.5rem;color:#dfedf7;font-family:var(--font-display);font-size:10rem;font-weight:900;line-height:1;content:attr(data-number)}
.subject-card>*{position:relative;z-index:1}
.subject-top{display:flex;align-items:center;justify-content:space-between;gap:.75rem;color:var(--blue);font-size:.72rem;font-weight:900;letter-spacing:.11em;text-transform:uppercase}
.subject-card h2{max-width:12ch;margin:auto 0 .7rem;color:var(--navy);font-family:var(--font-display);font-size:clamp(2.4rem,5vw,4rem);line-height:.86;text-transform:uppercase}
.subject-card p{max-width:32rem;margin:0;color:var(--muted);font-size:.9rem;line-height:1.5}
.subject-cta{margin-top:1rem;color:var(--navy);font-size:.8rem;font-weight:900;text-transform:uppercase}
.hub-note{display:grid;margin-top:1rem;padding:1.1rem 1.2rem;border:1px solid #bad3e6;border-radius:16px;background:#e8f5fd;grid-template-columns:auto minmax(0,1fr);gap:.8rem;color:#28445d;font-size:.9rem;line-height:1.5}
.hub-note strong{color:var(--navy)}
@media(max-width:820px){
  .reader-shell{grid-template-columns:1fr}
  .index-toggle{display:inline-flex}
  .back-link span{display:none}
  .reader-index{position:fixed;z-index:40;inset:68px 0 0;max-height:none;border:0;border-radius:0;transform:translateX(-105%);transition:transform .22s ease}
  body.index-open{overflow:hidden}
  body.index-open .reader-index{transform:translateX(0)}
  .concept-list{padding-bottom:5rem}
  .subject-grid{grid-template-columns:1fr}
}
@media(max-width:620px){
  body{background:#f4f8fc}
  .topbar-inner{width:calc(100% - 1rem);min-height:62px;gap:.5rem}
  .book-title strong{font-size:1.05rem}
  .book-title span{font-size:.6rem}
  .reader-index{inset:62px 0 0}
  .reader-shell{width:100%;margin:.65rem auto 6.3rem;gap:.65rem}
  .progress-card{margin:0 .65rem .65rem;padding:.65rem .75rem}
  .progress-card>span:first-child{display:none}
  .progress-card{grid-template-columns:minmax(0,1fr) auto}
  .concept-article{border-width:1px 0;border-radius:0;box-shadow:none}
  .concept-hero{min-height:44vh;display:flex;flex-direction:column;justify-content:flex-end;padding:1.45rem 1.15rem 1.55rem}
  .concept-step{align-self:flex-start;margin-bottom:.85rem}
  .concept-hero h1{font-size:clamp(2.7rem,15vw,4.6rem)}
  .concept-summary{font-size:1rem;line-height:1.58}
  .concept-body{padding:1.3rem 1.05rem 2rem}
  .reading-copy{font-size:1.04rem;line-height:1.75}
  .reading-copy h2{font-size:1.72rem}
  .mnemonic-grid{grid-template-columns:1fr}
  .concept-actions{margin-bottom:.5rem;grid-template-columns:1fr}
  .concept-actions .action-primary,.concept-actions .action-secondary{display:none}
  .question-note{display:none}
  .mobile-actions{position:fixed;z-index:25;right:0;bottom:0;left:0;display:grid;padding:.62rem max(.65rem,env(safe-area-inset-right)) calc(.62rem + env(safe-area-inset-bottom)) max(.65rem,env(safe-area-inset-left));border-top:1px solid #b6cbdd;background:rgba(255,255,255,.97);grid-template-columns:1fr 1fr;gap:.55rem;box-shadow:0 -12px 30px rgba(8,45,95,.14);backdrop-filter:blur(12px)}
  .mobile-actions .action-primary,.mobile-actions .action-secondary{min-height:48px;padding:.6rem .7rem;font-size:.82rem}
  .hub-body{background:linear-gradient(180deg,#072e61 0,#0f5b9e 25rem,#edf6fc 25rem,#f5f9fd 100%)}
  .hub-brand nav{display:none}
  .hub-hero{margin:1.8rem auto 1.6rem}
  .hub-hero h1{font-size:clamp(3.8rem,18vw,6rem)}
  .hub-main{width:calc(100% - 1rem)}
  .subject-card{min-height:230px;padding:1.15rem;border-radius:18px}
  .subject-card:nth-child(even){margin-left:1.1rem}
  .subject-card:nth-child(odd){margin-right:1.1rem}
  .hub-note{grid-template-columns:1fr}
}
@media(prefers-reduced-motion:reduce){html{scroll-behavior:auto}.reader-index,.progress-track i,.subject-card{transition:none}}