:root{
  --bg:#f4f7fb;
  --bg-soft:#eef3f8;
  --surface:#ffffff;
  --surface-soft:#f8fafc;
  --text:#182230;
  --muted:#667085;
  --primary:#2563eb;
  --primary-dark:#1d4ed8;
  --secondary:#0f766e;
  --accent:#f59e0b;
  --border:#dfe5ec;
  --code:#101828;
  --side:320px;
  --top:72px;
  --tech:46px;
  --shadow-sm:0 6px 18px rgba(15,23,42,.06);
  --shadow:0 18px 50px rgba(15,23,42,.10);
  --radius:18px;
}

body.dark{
  --bg:#0f172a;
  --bg-soft:#111c31;
  --surface:#172033;
  --surface-soft:#1c2940;
  --text:#f8fafc;
  --muted:#a8b3c7;
  --primary:#60a5fa;
  --primary-dark:#3b82f6;
  --secondary:#2dd4bf;
  --accent:#fbbf24;
  --border:#2b3a55;
  --code:#0b1220;
  --shadow-sm:0 6px 18px rgba(0,0,0,.18);
  --shadow:0 18px 50px rgba(0,0,0,.28);
}

*{box-sizing:border-box}
html{scroll-behavior:smooth}

body{
  margin:0;
  font-family:Inter,ui-sans-serif,system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif;
  background:
    radial-gradient(circle at top left,rgba(37,99,235,.06),transparent 28%),
    linear-gradient(180deg,var(--bg),var(--bg-soft));
  color:var(--text);
  line-height:1.72;
}

a{text-decoration:none;color:inherit}
button,input,select{font:inherit}

.topbar{
  position:fixed;
  inset:0 0 auto;
  height:var(--top);
  background:color-mix(in srgb,var(--surface) 94%,transparent);
  backdrop-filter:blur(18px);
  border-bottom:1px solid var(--border);
  display:flex;
  align-items:center;
  gap:24px;
  padding:0 20px;
  z-index:1000;
}

.brand{
  display:flex;
  align-items:center;
  gap:12px;
}

.brand strong{
  display:block;
  font-size:15px;
  letter-spacing:-.01em;
}

.brand small{
  display:block;
  font-size:12px;
  color:var(--muted);
}

.logo{
  width:44px;
  height:44px;
  border-radius:14px;
  background:linear-gradient(135deg,var(--primary),#7c3aed);
  color:#fff;
  display:grid;
  place-items:center;
  font-weight:800;
  box-shadow:0 10px 25px rgba(37,99,235,.25);
}

.topbar nav{
  display:flex;
  gap:8px;
}

.topbar nav a{
  padding:9px 13px;
  border-radius:10px;
  color:var(--muted);
  font-weight:700;
}

.topbar nav a:hover{
  background:var(--surface-soft);
  color:var(--primary);
}

.topbar .icon:last-child{margin-left:auto}

.icon{
  width:42px;
  height:42px;
  border:1px solid var(--border);
  background:var(--surface);
  color:var(--text);
  border-radius:12px;
  cursor:pointer;
  box-shadow:var(--shadow-sm);
}

.mobile{display:none}

.techbar{
  position:fixed;
  top:var(--top);
  left:0;
  right:0;
  height:var(--tech);
  background:#0f172a;
  color:#e2e8f0;
  display:flex;
  align-items:center;
  gap:10px;
  padding:0 18px;
  overflow:auto;
  z-index:990;
  scrollbar-width:none;
}

.techbar::-webkit-scrollbar{display:none}

.techbar a{
  white-space:nowrap;
  padding:7px 12px;
  border-radius:999px;
  color:#cbd5e1;
  font-size:13px;
  font-weight:700;
}

.techbar a:hover,
.techbar a.active{
  background:#1e293b;
  color:#7dd3fc;
}

.sidebar{
  position:fixed;
  left:0;
  top:calc(var(--top) + var(--tech));
  width:var(--side);
  height:calc(100vh - var(--top) - var(--tech));
  background:var(--surface);
  border-right:1px solid var(--border);
  overflow:auto;
  z-index:900;
}

.side-head{
  position:sticky;
  top:0;
  background:color-mix(in srgb,var(--surface) 96%,transparent);
  backdrop-filter:blur(12px);
  padding:16px;
  border-bottom:1px solid var(--border);
  z-index:2;
}

.side-head select,
.side-head input{
  width:100%;
  padding:12px 13px;
  border:1px solid var(--border);
  border-radius:12px;
  background:var(--surface-soft);
  color:var(--text);
  outline:none;
  margin-bottom:9px;
}

.side-head select:focus,
.side-head input:focus{
  border-color:var(--primary);
  box-shadow:0 0 0 4px rgba(37,99,235,.10);
}

.topic-link{
  display:block;
  padding:11px 16px;
  color:var(--muted);
  font-size:14px;
  border-bottom:1px solid color-mix(in srgb,var(--border) 55%,transparent);
  transition:.2s ease;
}

.topic-link:hover,
.topic-link.active{
  color:var(--primary);
  background:linear-gradient(90deg,rgba(37,99,235,.10),transparent);
  font-weight:700;
  padding-left:20px;
}

.main{
  margin-left:var(--side);
  padding-top:calc(var(--top) + var(--tech));
  min-height:100vh;
}

.hero{
  padding:74px clamp(24px,5vw,76px);
  background:
    linear-gradient(135deg,rgba(37,99,235,.13),rgba(15,118,110,.08)),
    var(--surface);
  border-bottom:1px solid var(--border);
}

.hero h1{
  max-width:900px;
  font-size:clamp(42px,5.8vw,74px);
  line-height:1.04;
  letter-spacing:-.045em;
  margin:10px 0 18px;
}

.hero p{
  font-size:18px;
  color:var(--muted);
  max-width:860px;
}

.eyebrow{
  display:inline-flex;
  align-items:center;
  gap:8px;
  color:var(--primary);
  font-size:12px;
  font-weight:800;
  letter-spacing:.14em;
}

.eyebrow::before{
  content:"";
  width:24px;
  height:2px;
  background:var(--primary);
  border-radius:99px;
}

.btn{
  display:inline-flex;
  align-items:center;
  justify-content:center;
  padding:12px 18px;
  border-radius:12px;
  background:var(--primary);
  color:#fff;
  font-weight:800;
  margin:14px 10px 0 0;
  box-shadow:0 10px 24px rgba(37,99,235,.20);
  transition:.2s ease;
}

.btn:hover{
  transform:translateY(-1px);
  background:var(--primary-dark);
}

.btn.secondary{
  background:var(--surface);
  color:var(--text);
  border:1px solid var(--border);
  box-shadow:var(--shadow-sm);
}

.section{
  padding:58px clamp(24px,5vw,76px);
}

.section h2{
  font-size:clamp(30px,4vw,48px);
  letter-spacing:-.025em;
  margin-bottom:28px;
}

.grid{
  display:grid;
  grid-template-columns:repeat(3,minmax(0,1fr));
  gap:22px;
}

.card,
.content-card{
  background:var(--surface);
  border:1px solid var(--border);
  border-radius:var(--radius);
  padding:24px;
  box-shadow:var(--shadow-sm);
}

.card{
  position:relative;
  overflow:hidden;
  transition:.25s ease;
}

.card::before{
  content:"";
  position:absolute;
  inset:0 auto 0 0;
  width:4px;
  background:linear-gradient(var(--primary),var(--secondary));
  opacity:0;
  transition:.25s ease;
}

.card:hover{
  transform:translateY(-4px);
  box-shadow:var(--shadow);
}

.card:hover::before{opacity:1}

.card h3{
  margin:0 0 10px;
  font-size:20px;
  letter-spacing:-.02em;
}

.card p,
.content-card p{
  color:var(--muted);
}

.card a{
  display:inline-flex;
  align-items:center;
  margin-top:8px;
  color:var(--primary);
  font-weight:800;
}

.tag{
  display:inline-block;
  font-size:12px;
  background:var(--surface-soft);
  color:var(--primary);
  border:1px solid var(--border);
  padding:5px 9px;
  border-radius:999px;
  margin:3px 3px 0 0;
}

.lesson-wrap{
  padding:46px clamp(24px,5vw,76px) 70px;
  max-width:1180px;
}

.breadcrumb{
  font-size:13px;
  color:var(--muted);
  margin-bottom:14px;
}

.lesson-wrap h1{
  font-size:clamp(38px,4.8vw,60px);
  line-height:1.08;
  letter-spacing:-.04em;
  margin:8px 0 10px;
}

.meta{
  color:var(--muted);
  font-size:13px;
}

.nav-buttons{
  display:flex;
  justify-content:space-between;
  gap:12px;
  margin:24px 0;
}

.nav-btn{
  padding:10px 14px;
  border:1px solid var(--border);
  background:var(--surface);
  border-radius:11px;
  font-weight:700;
  box-shadow:var(--shadow-sm);
}

.nav-btn:hover{
  color:var(--primary);
  border-color:var(--primary);
}

.content-card{
  margin-bottom:20px;
}

.content-card h2{
  margin:0 0 14px;
  font-size:24px;
  letter-spacing:-.02em;
}

.definition{
  border-left:5px solid var(--primary);
  background:linear-gradient(135deg,var(--surface),var(--surface-soft));
}

pre{
  background:var(--code);
  color:#eef4ff;
  padding:20px;
  border-radius:14px;
  overflow:auto;
  box-shadow:inset 0 1px 0 rgba(255,255,255,.05);
}

pre code{
  font-family:"JetBrains Mono","SFMono-Regular",Consolas,monospace;
  font-size:14px;
  line-height:1.75;
}

.columns{
  display:grid;
  grid-template-columns:1fr 1fr;
  gap:18px;
}

.qa{
  border:1px solid var(--border);
  padding:16px;
  border-radius:12px;
  margin:12px 0;
  background:var(--surface-soft);
}

.qa strong{
  display:block;
  margin-bottom:5px;
}

.callout{
  background:linear-gradient(135deg,rgba(245,158,11,.10),rgba(37,99,235,.06));
  border:1px solid color-mix(in srgb,var(--accent) 28%,var(--border));
  border-radius:12px;
  padding:16px;
  margin-top:14px;
}

.footer{
  margin-left:var(--side);
  background:#0f172a;
  color:#e5e7eb;
  display:grid;
  grid-template-columns:2fr 1fr 1fr;
  gap:28px;
  padding:42px clamp(24px,5vw,76px);
}

.footer h3{margin-top:0}
.footer p{color:#cbd5e1}

.overlay{display:none}

@media(max-width:1100px){
  .grid{grid-template-columns:repeat(2,minmax(0,1fr))}
  .topbar nav{display:none}
}

@media(max-width:850px){
  .mobile{display:block}
  .brand small{display:none}
  .sidebar{
    transform:translateX(-105%);
    transition:.25s ease;
    box-shadow:var(--shadow);
  }
  .sidebar.open{transform:translateX(0)}
  .main,.footer{margin-left:0}
  .overlay.show{
    display:block;
    position:fixed;
    inset:calc(var(--top) + var(--tech)) 0 0;
    background:rgba(15,23,42,.52);
    z-index:850;
  }
}

@media(max-width:650px){
  .topbar{padding:0 12px}
  .grid,.columns,.footer{grid-template-columns:1fr}
  .hero,.section,.lesson-wrap{padding-left:16px;padding-right:16px}
  .techbar{gap:6px}
  .hero h1{font-size:42px}
  .hero p{font-size:16px}
  .content-card{padding:18px}
  .nav-buttons{align-items:stretch}
  .nav-btn{font-size:13px}
}