:root{
  --bg:#f4f7fb;
  --bg-2:#eef3f9;
  --card:#ffffff;
  --text:#1f2937;
  --muted:#64748b;
  --accent:#6d5efc;
  --accent-2:#0ea5e9;
  --accent-soft:#eef2ff;
  --border:#dbe4f0;
  --shadow:0 14px 40px rgba(15,23,42,.08);
  --shadow-soft:0 8px 24px rgba(15,23,42,.05);
}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{
  margin:0;
  font-family:Arial,Helvetica,sans-serif;
  background:linear-gradient(180deg,var(--bg) 0%, var(--bg-2) 100%);
  color:var(--text);
  line-height:1.9;
}
a{color:var(--accent);text-underline-offset:3px}
.topbar{
  position:sticky;top:0;z-index:1000;
  background:rgba(255,255,255,.88);
  backdrop-filter:blur(14px);
  border-bottom:1px solid rgba(219,228,240,.9);
}
.topbar-inner{
  max-width:1200px;margin:0 auto;padding:14px 18px;
  display:flex;align-items:center;justify-content:space-between;gap:16px;flex-wrap:wrap;
}
.logo{
  color:#0f172a;text-decoration:none;font-weight:800;font-size:1.15rem;
}
.nav-links{display:flex;gap:10px;flex-wrap:wrap}
.nav-links a{
  text-decoration:none;padding:10px 16px;border-radius:999px;background:#fff;
  color:#334155;border:1px solid var(--border);font-weight:700;font-size:.95rem;
  box-shadow:var(--shadow-soft);
}
.nav-links a:hover{background:var(--accent-soft);border-color:#c7d2fe}
.nav-links a.nav-accent{background:linear-gradient(135deg,var(--accent) 0%, var(--accent-2) 100%);color:#fff;border-color:transparent}
.hero{max-width:1200px;margin:0 auto;padding:42px 18px 20px}
.hero-box{
  background:linear-gradient(135deg,#ffffff 0%, #f8fbff 52%, #eef2ff 100%);
  border:1px solid var(--border);border-radius:30px;padding:32px;box-shadow:var(--shadow);
}
.hero h1{margin:0 0 14px;font-size:2.2rem;color:#0f172a;line-height:1.35}
.hero p{margin:0 0 14px;font-size:1.06rem;color:#334155}
.hero-actions{display:flex;gap:12px;flex-wrap:wrap;margin-top:18px}
.button{
  display:inline-block;text-decoration:none;padding:12px 20px;border-radius:999px;
  font-weight:700;border:1px solid var(--border);box-shadow:var(--shadow-soft)
}
.button.primary{background:linear-gradient(135deg,var(--accent) 0%, var(--accent-2) 100%);color:#fff;border-color:transparent}
.button.secondary{background:#fff;color:#334155}
.layout{
  max-width:1200px;margin:0 auto;padding:0 18px 40px;
  display:grid;grid-template-columns:minmax(0,1fr) 320px;gap:24px;align-items:start;
}
.content,.sidebar,details.mobile-lessons{
  background:var(--card);border-radius:24px;padding:28px;border:1px solid var(--border);box-shadow:var(--shadow);
}
.content h2{font-size:1.6rem;color:#0f172a;margin:30px 0 12px}
.content h3{font-size:1.18rem;color:#1e293b;margin:22px 0 10px}
.content p{margin:0 0 14px}
.content ul,.content ol{padding-right:22px;margin:0 0 18px}
.content li{margin-bottom:8px}
.note,.cta-box{
  background:linear-gradient(180deg,#f8fbff 0%, #f5f3ff 100%);
  border:1px solid var(--border);border-radius:18px;padding:18px;margin:22px 0;
}
.sidebar h3{margin-top:0;color:#0f172a}
.lesson-list{list-style:none;padding:0;margin:0}
.lesson-list li{margin:0 0 10px}
.lesson-list a{
  display:block;text-decoration:none;background:#f8fafc;border:1px solid var(--border);
  padding:12px 14px;border-radius:16px;color:#334155;font-weight:700;
}
.lesson-list a.active{background:linear-gradient(135deg,var(--accent) 0%, var(--accent-2) 100%);color:#fff;border-color:transparent}
.lesson-list a:hover{background:#eef2ff;border-color:#c7d2fe}
.breadcrumbs{font-size:.95rem;color:var(--muted);margin-bottom:10px}
.breadcrumbs a{text-decoration:none;color:var(--muted)}
.table-wrap{overflow:auto;margin:18px 0}
table{width:100%;border-collapse:collapse;background:#fff;border-radius:16px;overflow:hidden}
th,td{padding:12px 14px;border:1px solid var(--border);text-align:right;vertical-align:top}
th{background:#f8fafc;color:#0f172a}
.next-prev{display:flex;justify-content:space-between;gap:12px;flex-wrap:wrap;margin-top:34px;padding-top:22px;border-top:1px solid var(--border)}
.footer{max-width:1200px;margin:0 auto 40px;padding:0 18px;color:var(--muted);font-size:.95rem}
.mobile-lessons{display:none;margin:0 auto 18px;max-width:1200px}
.mobile-lessons summary{cursor:pointer;font-weight:800;color:#0f172a;list-style:none;display:flex;align-items:center;justify-content:space-between;gap:12px}
.mobile-lessons summary::-webkit-details-marker{display:none}
.mobile-lessons summary::after{content:"+";font-size:1.4rem;color:var(--accent)}
.mobile-lessons[open] summary::after{content:"−"}
.mobile-lessons .lesson-list{margin-top:16px}
@media (max-width: 980px){
  .layout{grid-template-columns:1fr}
  .sidebar{display:none}
  .mobile-lessons{display:block}
  .hero h1{font-size:1.8rem}
}
@media (max-width: 640px){
  .topbar-inner{justify-content:center}
  .hero-box,.content,.sidebar,.mobile-lessons{padding:20px}
  .hero h1{font-size:1.55rem}
  .nav-links{justify-content:center}
  .nav-links a{flex:1 1 calc(50% - 10px);text-align:center}
  .button{width:100%;text-align:center}
}