:root{
  --s1:#1d7a4f; --s2:#b3491c; --s3:#c8930c; --s4:#2456a5;
  --ink:#1c2630; --muted:#5a6b78; --bg:#f4f6f8; --card:#fff; --line:#e1e7ed;
  --accent:#2456a5; --shadow:0 1px 3px rgba(16,30,45,.08),0 4px 14px rgba(16,30,45,.06);
}
*{box-sizing:border-box}
html,body{margin:0;padding:0}
body{font-family:"Segoe UI",system-ui,Roboto,Helvetica,Arial,sans-serif;color:var(--ink);background:var(--bg);line-height:1.5}
a{color:var(--accent);text-decoration:none}
a:hover{text-decoration:underline}
h1,h2,h3{line-height:1.25;margin:.3em 0}
img{max-width:100%;height:auto}
code,.mono{font-family:ui-monospace,Consolas,monospace}

/* top bar */
.topbar{background:#0f2740;color:#fff;position:sticky;top:0;z-index:60;box-shadow:var(--shadow)}
.topbar-inner{display:flex;align-items:center;gap:12px;max-width:1200px;margin:0 auto;padding:10px 14px;flex-wrap:wrap}
.brand{color:#fff;font-weight:700;font-size:1.05rem;white-space:nowrap}
.brand .logo{color:#ffd24a}
.brand .yr{background:#ffd24a;color:#0f2740;border-radius:4px;padding:0 5px;font-size:.75rem;vertical-align:middle}
.menu-btn{display:none;background:none;border:1px solid rgba(255,255,255,.4);color:#fff;font-size:1.2rem;border-radius:6px;padding:4px 10px}
.search-box{flex:1;display:flex;min-width:180px;max-width:360px;background:#fff;border-radius:20px;overflow:hidden}
.search-box input{flex:1;border:0;padding:8px 12px;font-size:.9rem;outline:none}
.search-box button{border:0;background:transparent;padding:0 12px;cursor:pointer;font-size:1rem}
.mainnav{display:flex;gap:6px;flex-wrap:wrap;width:100%}
.mainnav a{color:#e6eef5;padding:5px 9px;border-radius:6px;font-size:.88rem}
.mainnav a:hover{background:rgba(255,255,255,.15);text-decoration:none}
.mainnav a.active{background:#ffd24a;color:#0f2740;font-weight:600}

/* layout */
.layout{display:grid;grid-template-columns:250px 1fr;max-width:1200px;margin:0 auto;gap:18px;padding:18px 14px}
.sidebar{position:sticky;top:64px;align-self:start;max-height:calc(100vh - 80px);overflow:auto}
.sidebar h4{text-transform:uppercase;letter-spacing:.04em;font-size:.72rem;color:var(--muted);margin:14px 0 6px}
.sidebar a{display:block;padding:6px 10px;border-radius:6px;color:var(--ink)}
.sidebar a:hover{background:#eef2f6;text-decoration:none}
.sidebar a.active{background:var(--accent);color:#fff}
.c-s1{--accent:var(--s1)} .c-s2{--accent:var(--s2)} .c-s3{--accent:var(--s3)} .c-s4{--accent:var(--s4)}

/* cards */
.card{background:var(--card);border:1px solid var(--line);border-radius:12px;padding:18px;box-shadow:var(--shadow)}
.grid{display:grid;gap:14px}
.g2{grid-template-columns:repeat(2,1fr)} .g3{grid-template-columns:repeat(3,1fr)} .g4{grid-template-columns:repeat(4,1fr)}
@media(max-width:900px){.g4{grid-template-columns:repeat(2,1fr)}.g3{grid-template-columns:repeat(2,1fr)}}
@media(max-width:620px){.g2,.g3,.g4{grid-template-columns:1fr}.layout{grid-template-columns:1fr}.sidebar{display:none}.mainnav{display:none}.menu-btn{display:block}}

/* strand cards */
.strand-card{border-radius:14px;color:#fff;padding:20px;box-shadow:var(--shadow);position:relative;overflow:hidden;cursor:pointer;border:0;text-align:left}
.strand-card h3{margin:.1em 0 .2em;font-size:1.35rem}
.strand-card .icon{font-size:2.3rem}
.strand-card .meta{font-size:.85rem;opacity:.92}
.strand-card small{display:inline-block;margin-top:10px;background:rgba(255,255,255,.22);padding:4px 10px;border-radius:12px;font-weight:600}

/* buttons */
.btn{display:inline-block;border:1px solid var(--line);background:#fff;color:var(--ink);padding:8px 16px;border-radius:8px;cursor:pointer;font-size:.9rem;text-align:center}
.btn:hover{text-decoration:none;background:#f0f3f6}
.btn.primary{background:var(--accent);color:#fff;border-color:transparent}
.btn.primary:hover{filter:brightness(1.05);background:var(--accent)}
.btn.warn{background:#c0392b;color:#fff;border-color:transparent}
.btn.success{background:var(--s1);color:#fff;border-color:transparent}
.btn.small{padding:4px 10px;font-size:.8rem}
.btn.small:disabled{opacity:.5;cursor:not-allowed}
.btn.red{background:var(--s2);color:#fff;border-color:transparent}

/* quiz / mcq options */
.opt{display:block;text-align:left;width:100%;border:1px solid var(--line);background:#fff;border-radius:8px;padding:10px 12px;margin:6px 0;cursor:pointer;font-size:.95rem}
.opt:hover{border-color:var(--accent)}
.opt.sel{border-color:var(--accent);background:#eef4ff}
.opt.ok{border-color:var(--s1);background:#e6f6ec}
.opt.bad{border-color:var(--s2);background:#fdecec}

/* tags */
.tag{display:inline-block;background:#eef2f6;border-radius:10px;padding:2px 9px;font-size:.72rem;margin:2px 3px 2px 0;color:#33445a}
.tag.t-s1{background:#e4f2ea;color:var(--s1)} .tag.t-s2{background:#fbe7dc;color:var(--s2)}
.tag.t-s3{background:#fbf0d2;color:#8a6a05} .tag.t-s4{background:#e3ebfa;color:var(--s4)}
.diff{font-size:.72rem;font-weight:700;border-radius:6px;padding:1px 7px}
.d1{background:#e6f6ec;color:var(--s1)}.d2{background:#fff4cc;color:#8a6a05}.d3{background:#fde6d9;color:var(--s2)}.d4{background:#fbe0e0;color:#a02020}

/* tables */
table{width:100%;border-collapse:collapse;background:#fff;font-size:.88rem}
th,td{border:1px solid var(--line);padding:7px 9px;text-align:left;vertical-align:top}
th{background:#eef2f6}
.tablewrap{overflow:auto;border-radius:10px;border:1px solid var(--line)}

/* progress */
.bar{height:10px;border-radius:6px;background:#e6ebf0;overflow:hidden}
.bar>span{display:block;height:100%;background:var(--accent);border-radius:6px}
.pill{display:inline-block;padding:2px 9px;border-radius:12px;background:#eef2f6;font-size:.75rem;font-weight:600}

/* note layout */
.note h3{border-bottom:2px solid var(--line);padding-bottom:4px}
.note .lbl{display:inline-block;font-weight:700;text-transform:uppercase;font-size:.7rem;letter-spacing:.04em;color:var(--accent);margin-top:10px}
.keywrap .k{font-weight:600}
.bbox{background:#fff8e1;border-left:4px solid #f0b400;padding:8px 12px;border-radius:0 8px 8px 0;margin:8px 0}
.warnbox{background:#fdecec;border-left:4px solid var(--s2);padding:8px 12px;border-radius:0 8px 8px 0;margin:8px 0}
.tipbox{background:#eef4ff;border-left:4px solid var(--s4);padding:8px 12px;border-radius:0 8px 8px 0;margin:8px 0}

/* filters */
.filters{display:flex;flex-wrap:wrap;gap:10px;align-items:center;margin:12px 0}
.filters select,.filters input{padding:7px 9px;border:1px solid var(--line);border-radius:8px;background:#fff;font-size:.9rem}
.filters .lbl{font-size:.8rem;color:var(--muted)}

/* simulator */
.timer{font-size:1.4rem;font-weight:700;color:var(--s2);font-variant-numeric:tabular-nums}
.stickybar{position:sticky;top:60px;z-index:40;background:#fff;border:1px solid var(--line);border-radius:10px;padding:10px 14px;box-shadow:var(--shadow);display:flex;gap:14px;align-items:center;flex-wrap:wrap;margin-bottom:14px}

/* svg graph */
svg.graph{width:100%;height:auto;border:1px solid var(--line);border-radius:10px;background:#fff}
.axis-t,.axis-l{font-size:12px;fill:#33445a}
.axis-lab{font-size:13px;font-weight:600;fill:#1c2630}

/* modal */
.modal{position:fixed;inset:0;background:rgba(15,35,55,.55);display:none;align-items:center;justify-content:center;z-index:100;padding:16px}
.modal.open{display:flex}
.modal-card{background:#fff;border-radius:12px;max-width:920px;width:100%;max-height:92vh;overflow:auto;padding:20px}
.modal .close{float:right;background:none;border:0;font-size:1.5rem;cursor:pointer;color:#33445a}

/* toast */
.toast{position:fixed;bottom:20px;left:50%;transform:translateX(-50%);background:#0f2740;color:#fff;padding:10px 18px;border-radius:8px;opacity:0;pointer-events:none;transition:.3s;z-index:120}
.toast.show{opacity:1}

/* footer */
.foot{background:#0f2740;color:#cfe0ee;text-align:center;padding:18px 14px;font-size:.85rem;margin-top:30px}
.foot a{color:#ffd24a}

/* hero */
.hero{background:linear-gradient(135deg,#0f2740,#2456a5);color:#fff;border-radius:16px;padding:26px;margin-bottom:18px}
.hero h1{margin:0 0 6px;font-size:1.8rem}
.hero p{margin:.2em 0;color:#dbe7f2}
.hero .quick{display:flex;flex-wrap:wrap;gap:10px;margin-top:14px}

.breadcrumb{font-size:.85rem;color:var(--muted);margin-bottom:10px}
.muted{color:var(--muted)}
.section-title{display:flex;align-items:center;justify-content:space-between;margin:22px 0 8px}
.section-title h2{margin:0}
hr.rule{border:0;border-top:1px solid var(--line);margin:16px 0}
.notice{background:#fff;border:1px solid var(--line);border-left:4px solid var(--accent);padding:10px 14px;border-radius:0 8px 8px 0;font-size:.9rem;color:#33445a}