:root {
  --lesson-bg: #020904;
  --lesson-surface: rgba(6, 25, 11, 0.74);
  --lesson-surface-strong: #030d06;
  --lesson-green: #00e676;
  --lesson-border: rgba(0, 230, 118, 0.15);
  --lesson-text: #eef7f0;
  --lesson-secondary: #aebdb1;
  --lesson-muted: #66806c;
  --lesson-warning: #ffab40;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  min-height: 100vh;
  overflow-x: hidden;
  background:
    radial-gradient(circle at 18% 8%, rgba(0,230,118,0.055), transparent 28%),
    radial-gradient(circle at 82% 30%, rgba(20,120,65,0.035), transparent 32%),
    var(--lesson-bg);
  color: var(--lesson-text);
  font-family: 'Inter','Noto Sans SC',-apple-system,BlinkMacSystemFont,'Segoe UI',sans-serif;
  line-height: 1.75;
  -webkit-font-smoothing: antialiased;
}
a { color: inherit; }

.site-nav {
  position: relative;
  z-index: 1000;
  background: rgba(2,9,4,.94);
  border-bottom: 1px solid var(--lesson-border);
  backdrop-filter: blur(18px);
}
.nav-container { max-width: 1280px; height: 68px; margin: 0 auto; padding: 0 24px; display: flex; align-items: center; justify-content: space-between; gap: 18px; }
.nav-left { display: flex; align-items: center; gap: 12px; flex-shrink: 0; }
.logo-icon img { display: block; width: 40px; height: 40px; border-radius: 10px; }
.logo-text { display: flex; flex-direction: column; }
.logo-title { color: var(--lesson-text); font-size: 16px; font-weight: 700; line-height: 1.3; }
.logo-sub { color: var(--lesson-muted); font-size: 11px; line-height: 1.3; }
.nav-links { display: flex; align-items: center; gap: 4px; }
.nav-link { padding: 8px 12px; border-radius: 8px; color: #aab8ad; font-size: 14px; font-weight: 500; text-decoration: none; white-space: nowrap; transition: .2s ease; }
.nav-link:hover, .nav-link.active { color: var(--lesson-green); background: rgba(0,230,118,.08); }
.nav-right { display: flex; align-items: center; gap: 10px; flex-shrink: 0; }
.btn-primary { padding: 8px 17px; border: 0; border-radius: 8px; background: linear-gradient(135deg,#00e676,#14783d); color: #001b0c; font: inherit; font-size: 13px; font-weight: 700; cursor: pointer; }
.nav-toggle { display: none; width: 40px; height: 40px; align-items: center; justify-content: center; border: 1px solid rgba(0,230,118,.28); border-radius: 10px; background: rgba(0,230,118,.08); color: var(--lesson-green); font-size: 21px; cursor: pointer; }
.lesson-page { max-width: 1180px; margin: 0 auto; padding: 36px 24px 60px; }
.lesson-breadcrumb { display: flex; align-items: center; flex-wrap: wrap; gap: 8px; margin-bottom: 24px; color: var(--lesson-muted); font-size: 12px; }
.lesson-breadcrumb a { text-decoration: none; }
.lesson-breadcrumb a:hover { color: var(--lesson-green); }
.lesson-breadcrumb strong { color: #a7b7aa; font-weight: 500; }

.lesson-heading { padding-bottom: 30px; border-bottom: 1px solid var(--lesson-border); }
.lesson-label { display: inline-flex; align-items: center; gap: 7px; padding: 4px 10px; border: 1px solid rgba(0,230,118,.22); border-radius: 7px; background: rgba(0,230,118,.07); color: var(--lesson-green); font-size: 12px; font-weight: 600; }
.lesson-label.intermediate { border-color: rgba(38,198,218,.26); background: rgba(38,198,218,.08); color: #35c9dc; }
.lesson-label.expert { border-color: rgba(255,82,82,.24); background: rgba(255,82,82,.08); color: #ff5d65; }
.lesson-heading h1 { margin: 13px 0 7px; font-size: clamp(34px,5vw,48px); line-height: 1.18; letter-spacing: -.035em; }
.lesson-lead { max-width: 760px; margin: 0; color: #99ab9d; font-size: 16px; }
.lesson-meta { display: flex; flex-wrap: wrap; gap: 8px 20px; margin-top: 18px; color: var(--lesson-muted); font-size: 12px; }

.lesson-layout { display: grid; grid-template-columns: minmax(0,1fr) 250px; gap: 46px; align-items: start; padding-top: 34px; }
.lesson-article { min-width: 0; }
.lesson-article section { margin-bottom: 38px; scroll-margin-top: 24px; }
.lesson-article h2 { margin: 0 0 14px; font-size: 23px; line-height: 1.35; }
.lesson-article h3 { margin: 0 0 8px; font-size: 16px; }
.lesson-article p { margin: 0 0 15px; color: var(--lesson-secondary); }
.lesson-article code { padding: 2px 6px; border: 1px solid var(--lesson-border); border-radius: 5px; background: rgba(0,230,118,.055); color: #8ef0b7; font-family: Consolas,'SFMono-Regular',monospace; font-size: .88em; overflow-wrap: anywhere; }
.lesson-opening { color: #dae6dd !important; font-size: 18px; line-height: 1.9; }
.lesson-keyline { margin: 22px 0 34px; padding: 17px 20px; border-left: 3px solid var(--lesson-green); background: linear-gradient(90deg,rgba(0,230,118,.1),rgba(0,230,118,.015)); color: #d9f8e2; font-size: 16px; }
.lesson-note { margin-top: 18px; padding: 16px 18px; border: 1px solid var(--lesson-border); border-radius: 11px; background: rgba(5,25,10,.54); color: #91a897; font-size: 13px; }
.lesson-note strong { color: #d8e8dc; }

.concept-grid { display: grid; grid-template-columns: repeat(3,minmax(0,1fr)); gap: 12px; margin-top: 18px; }
.concept-grid.two-column { grid-template-columns: repeat(2,minmax(0,1fr)); }
.concept-card { padding: 18px 17px; border-top: 1px solid rgba(0,230,118,.28); background: rgba(5,23,10,.62); }
.concept-card-index { margin-bottom: 9px; color: var(--lesson-green); font-family: Consolas,'SFMono-Regular',monospace; font-size: 14px; }
.concept-card p { margin: 0; color: #809685; font-size: 13px; }
.concept-card small { display: block; margin-bottom: 8px; color: #5f7865; }

.lesson-timeline { display: grid; grid-template-columns: repeat(3,minmax(0,1fr)); gap: 12px; margin-top: 20px; }
.timeline-step { position: relative; min-height: 164px; padding: 18px; border: 1px solid var(--lesson-border); border-radius: 12px; background: rgba(6,25,11,.68); }
.timeline-era { color: var(--lesson-green); font: 600 13px Consolas,'SFMono-Regular',monospace; }
.timeline-step h3 { margin-top: 10px; }
.timeline-step p { margin: 0; color: #809485; font-size: 13px; }

.lesson-table-wrap { overflow-x: auto; margin-top: 18px; border: 1px solid var(--lesson-border); border-radius: 12px; }
.lesson-table { width: 100%; min-width: 650px; border-collapse: collapse; background: rgba(5,20,9,.6); }
.lesson-table th, .lesson-table td { padding: 13px 14px; border-bottom: 1px solid rgba(255,255,255,.055); text-align: left; vertical-align: top; font-size: 13px; }
.lesson-table th { color: #e5f2e8; background: rgba(0,230,118,.055); font-weight: 600; }
.lesson-table th:first-child, .lesson-table td:first-child { color: #78907e; white-space: nowrap; }
.lesson-table td { color: #9cad9f; }
.lesson-table tr:last-child td { border-bottom: 0; }

.lesson-flow { display: grid; grid-template-columns: 1fr auto 1fr auto 1fr; gap: 8px; align-items: stretch; margin-top: 19px; }
.lesson-flow-node { padding: 17px 14px; border: 1px solid var(--lesson-border); border-radius: 10px; background: rgba(6,25,11,.68); }
.lesson-flow-node small { display: block; margin-top: 4px; color: #55705b; }
.lesson-flow-arrow { align-self: center; color: var(--lesson-green); }

.lesson-warning { padding: 18px 20px; border: 1px solid rgba(255,171,64,.22); border-radius: 11px; background: rgba(105,56,5,.12); }
.lesson-warning h3 { color: #ffb55c; }
.lesson-warning p { margin: 0; color: #c4aa86; }
.lesson-summary, .lesson-checklist { display: grid; gap: 11px; margin: 0; padding: 0; list-style: none; }
.lesson-summary li, .lesson-checklist li { display: grid; grid-template-columns: 24px 1fr; gap: 10px; color: #b8c6bb; }
.lesson-summary li::before { content: '✓'; color: var(--lesson-green); font-weight: 700; }
.lesson-checklist { padding: 18px 20px; border: 1px solid var(--lesson-border); border-radius: 12px; background: rgba(5,23,10,.58); }
.lesson-checklist li::before { content: '□'; color: var(--lesson-green); font-weight: 700; }
.lesson-steps { counter-reset: lesson-step; display: grid; gap: 12px; margin: 18px 0 0; padding: 0; list-style: none; }
.lesson-steps li { counter-increment: lesson-step; display: grid; grid-template-columns: 38px 1fr; gap: 13px; align-items: start; padding: 16px 17px; border: 1px solid var(--lesson-border); border-radius: 11px; background: rgba(5,23,10,.58); color: #aebdb1; }
.lesson-steps li::before { content: counter(lesson-step, decimal-leading-zero); display: flex; align-items: center; justify-content: center; width: 34px; height: 34px; border: 1px solid rgba(0,230,118,.24); border-radius: 50%; background: rgba(0,230,118,.08); color: var(--lesson-green); font: 600 12px Consolas,'SFMono-Regular',monospace; }
.lesson-steps strong { display: block; margin-bottom: 3px; color: #e4eee6; }
.lesson-steps small { display: block; color: #78907e; font-size: 12px; line-height: 1.65; }

.lesson-next { display: flex; align-items: center; justify-content: space-between; gap: 20px; padding: 21px 22px; border: 1px solid rgba(0,230,118,.2); border-radius: 12px; background: linear-gradient(135deg,rgba(5,38,16,.88),rgba(4,18,8,.9)); text-decoration: none; transition: .2s ease; }
.lesson-next:hover { border-color: rgba(0,230,118,.4); transform: translateY(-2px); }
.lesson-next small { display: block; margin-bottom: 3px; color: #5c7a63; }
.lesson-next strong { font-size: 17px; }
.lesson-next-arrow { color: var(--lesson-green); font-size: 24px; }

.lesson-sources { position: sticky; top: 20px; padding: 18px; border: 1px solid var(--lesson-border); border-radius: 12px; background: rgba(5,20,9,.78); }
.lesson-sources h2 { margin: 0 0 11px; font-size: 13px; }
.lesson-sources ul { margin: 0; padding: 0; list-style: none; }
.lesson-sources li { padding: 9px 0; border-bottom: 1px solid rgba(255,255,255,.04); font-size: 12px; }
.lesson-sources li:last-child { border-bottom: 0; }
.lesson-sources a { color: #82a18a; text-decoration: none; }
.lesson-sources a:hover { color: var(--lesson-green); }

.footer { margin-top: 54px; border-top: 1px solid var(--lesson-border); background: linear-gradient(180deg,transparent,rgba(0,0,0,.4)); }
.footer-inner { max-width: 1280px; margin: 0 auto; padding: 44px 24px 30px; display: grid; grid-template-columns: 1.6fr 1fr 1fr 1fr 1.2fr; gap: 38px; }
.footer-brand { display: flex; align-items: center; gap: 12px; margin-bottom: 14px; }
.footer-brand img { width: 44px; height: 44px; border-radius: 10px; }
.footer-desc { max-width: 300px; color: var(--lesson-muted); font-size: 13px; }
.footer-col h4, .footer-social-title { margin: 0 0 14px; font-size: 14px; }
.footer-col a { display: block; margin-bottom: 9px; color: var(--lesson-muted); font-size: 13px; text-decoration: none; }
.footer-col a:hover { color: var(--lesson-green); }
.social-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 8px; }
.social-item { display: flex; align-items: center; gap: 6px; padding: 8px 9px; border: 1px solid var(--lesson-border); border-radius: 9px; color: var(--lesson-muted); font-size: 12px; text-decoration: none; }
.social-item img { width: 18px; height: 18px; object-fit: contain; }
.footer-secondary { max-width: 1280px; margin: 0 auto; padding: 18px 24px 0; border-top: 1px solid rgba(0,230,118,.06); }
.footer-meta { display: flex; flex-direction: column; gap: 5px; margin-bottom: 13px; color: var(--lesson-muted); font-size: 11px; }
.footer-bottom { max-width: 1280px; margin: 0 auto; padding: 18px 24px 24px; display: flex; justify-content: space-between; gap: 16px; color: var(--lesson-muted); font-size: 11px; }
.back-to-top { position: fixed; right: 24px; bottom: 24px; width: 40px; height: 40px; border: 1px solid var(--lesson-border); border-radius: 10px; background: rgba(5,25,10,.92); color: var(--lesson-green); cursor: pointer; opacity: 0; visibility: hidden; transition: .2s ease; }
.back-to-top.visible { opacity: 1; visibility: visible; }

@media (max-width: 980px) {
  .nav-links { display: none; }
  .nav-toggle { display: flex; }
  .lesson-layout { grid-template-columns: 1fr; }
  .lesson-sources { position: static; max-width: 440px; }
  .footer-inner { grid-template-columns: repeat(2,minmax(0,1fr)); }
}
@media (max-width: 768px) {
  .nav-container { height: 58px; padding: 0 12px; }
  .nav-toggle { display: flex !important; }
  .nav-right .lang-dropdown { display: none !important; }
  .btn-primary { padding: 7px 12px; }
  .lesson-page { padding: 26px 14px 44px; }
  .lesson-heading h1 { font-size: 31px; }
  .lesson-layout { gap: 30px; padding-top: 28px; }
  .concept-grid, .concept-grid.two-column, .lesson-timeline { grid-template-columns: 1fr; }
  .lesson-flow { grid-template-columns: 1fr; }
  .lesson-flow-arrow { justify-self: center; transform: rotate(90deg); }
  .footer-inner { grid-template-columns: 1fr 1fr !important; padding: 30px 14px 20px !important; gap: 24px 18px !important; }
  .footer-bottom { padding: 16px 14px 22px; flex-direction: column; }
}
@media (max-width: 480px) {
  .logo-sub { display: none; }
  .btn-login { display: none !important; }
  .lesson-meta { flex-direction: column; gap: 5px; }
  .lesson-opening { font-size: 16px; }
  .footer-inner { grid-template-columns: 1fr !important; }
  .social-grid { max-width: 260px; }
}