:root {
  color-scheme: dark;
  --bg: #070b16;
  --panel: #0d1425;
  --panel-2: #111b30;
  --line: #24324a;
  --muted: #91a0b8;
  --text: #f3f7ff;
  --cyan: #2ee6c5;
  --cyan-soft: rgba(46, 230, 197, .12);
  --coral: #ff7f66;
  --yellow: #ffd45f;
  --danger: #ff6b85;
  --radius: 18px;
}

* { box-sizing: border-box; }
html { scroll-behavior: smooth; }
body {
  margin: 0;
  min-width: 320px;
  background:
    radial-gradient(circle at 75% -10%, rgba(46, 230, 197, .09), transparent 28rem),
    var(--bg);
  color: var(--text);
  font-family: "Pretendard", "Noto Sans KR", system-ui, sans-serif;
  font-size: 16px;
  line-height: 1.65;
  overflow-x: hidden;
}
button, input { font: inherit; }
button { color: inherit; }
.skip-link { position: fixed; left: 1rem; top: -10rem; z-index: 100; padding: .7rem 1rem; background: var(--cyan); color: #04110f; }
.skip-link:focus { top: 1rem; }
.sr-only { position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden; clip: rect(0,0,0,0); white-space: nowrap; border: 0; }

.app-shell { min-height: 100vh; display: grid; grid-template-columns: 290px minmax(0, 1fr); grid-template-rows: 76px 1fr; }
.topbar { grid-column: 1 / -1; position: sticky; top: 0; z-index: 30; display: grid; grid-template-columns: 290px 1fr auto; align-items: center; min-height: 76px; border-bottom: 1px solid var(--line); background: rgba(7, 11, 22, .9); backdrop-filter: blur(18px); }
.brand { height: 100%; display: flex; align-items: center; gap: .8rem; padding: 0 1.5rem; border-right: 1px solid var(--line); color: var(--text); text-decoration: none; font-weight: 800; letter-spacing: -.02em; }
.brand-mark { display: grid; place-items: center; width: 2.3rem; height: 2.3rem; border: 1px solid var(--cyan); border-radius: 10px; color: var(--cyan); font-family: ui-monospace, monospace; }
.topbar-stats { display: flex; justify-content: flex-end; align-items: center; gap: 1rem; padding: 0 1.5rem; color: var(--muted); font-size: .9rem; }
.progress-track { width: min(20vw, 210px); height: 6px; overflow: hidden; border-radius: 99px; background: #1b2740; }
.progress-track span { display: block; width: 0; height: 100%; background: linear-gradient(90deg, var(--cyan), #62a9ff); transition: width .35s ease; }
.mobile-menu { display: none; margin-right: 1rem; padding: .55rem .8rem; border: 1px solid var(--line); border-radius: 10px; background: var(--panel); }

.sidebar { position: sticky; top: 76px; height: calc(100vh - 76px); overflow-y: auto; border-right: 1px solid var(--line); background: rgba(10, 16, 31, .78); }
.sidebar-heading { padding: 1.5rem 1.25rem .8rem; }
.sidebar-heading h2 { margin: .1rem 0 0; font-size: 1.15rem; }
.eyebrow { margin: 0; color: var(--cyan); font: 700 .75rem/1.3 ui-monospace, SFMono-Regular, Consolas, monospace; letter-spacing: .13em; }
.search-box { position: relative; display: block; margin: 0 1rem 1rem; }
.search-box input { width: 100%; padding: .7rem 2.6rem .7rem .9rem; border: 1px solid var(--line); border-radius: 10px; outline: none; background: #090f1d; color: var(--text); }
.search-box input:focus { border-color: var(--cyan); box-shadow: 0 0 0 3px var(--cyan-soft); }
.search-box kbd { position: absolute; top: .55rem; right: .65rem; padding: .05rem .4rem; border: 1px solid var(--line); border-radius: 5px; color: var(--muted); background: var(--panel-2); }
.nav-group-title { padding: .9rem 1.25rem .45rem; color: #62708a; font-size: .72rem; font-weight: 800; letter-spacing: .11em; text-transform: uppercase; }
.topic-link { width: 100%; display: grid; grid-template-columns: 1.7rem 1fr auto; align-items: center; gap: .55rem; padding: .62rem 1.15rem; border: 0; border-left: 3px solid transparent; background: transparent; text-align: left; cursor: pointer; }
.topic-link:hover { background: rgba(255,255,255,.035); }
.topic-link.active { border-left-color: var(--cyan); background: var(--cyan-soft); color: var(--cyan); }
.topic-link .num { color: #64738c; font: .72rem ui-monospace, monospace; }
.topic-link .done { color: var(--cyan); font-size: .8rem; }

.main { width: min(1080px, 100%); margin: 0 auto; padding: 3.5rem 3rem 4rem; }
.topic-hero { display: flex; justify-content: space-between; gap: 2rem; align-items: flex-start; margin-bottom: 2rem; }
.topic-hero > div:first-child { min-width: 0; }
.topic-hero h1 { max-width: 760px; margin: .45rem 0 .65rem; font-size: clamp(2.1rem, 5vw, 4rem); line-height: 1.08; letter-spacing: -.045em; }
.topic-summary { max-width: 760px; margin: 0; color: #b4c0d4; font-size: 1.08rem; }
.topic-count { flex: 0 0 auto; display: grid; place-items: center; width: 74px; height: 74px; border: 1px solid var(--line); border-radius: 50%; color: var(--muted); font: 700 .9rem ui-monospace, monospace; }

.study-grid { display: grid; grid-template-columns: minmax(0, 1fr) 290px; gap: 1rem; align-items: start; }
.lesson-card, .memory-card, .quiz-section { border: 1px solid var(--line); border-radius: var(--radius); background: linear-gradient(145deg, rgba(17,27,48,.96), rgba(11,17,32,.96)); box-shadow: 0 20px 70px rgba(0,0,0,.18); }
.lesson-card { padding: 1.7rem; }
.section-kicker { display: flex; gap: .65rem; align-items: center; margin-bottom: 1.4rem; color: #a9b7cc; font-weight: 750; }
.section-kicker span { color: var(--cyan); font: 700 .75rem ui-monospace, monospace; }
.lesson-card h2 { margin: 1.4rem 0 .4rem; font-size: 1.25rem; }
.lesson-card p { margin: .35rem 0; color: #c6d0e0; }
.lesson-card ul { padding-left: 1.2rem; color: #c6d0e0; }
.lesson-card li + li { margin-top: .3rem; }
code { padding: .1em .35em; border-radius: 5px; background: #07101c; color: #81f1dc; font-family: ui-monospace, SFMono-Regular, Consolas, monospace; }
pre { overflow-x: auto; margin: 1rem 0; padding: 1rem; border: 1px solid #223452; border-radius: 12px; background: #070d19; }
pre code { padding: 0; background: none; color: #d7e2f4; font-size: .9rem; }
.concept-compare { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: .7rem; margin-top: 1rem; }
.concept-box { padding: 1rem; border: 1px solid var(--line); border-radius: 12px; background: rgba(5,10,20,.45); }
.concept-box strong { display: block; color: var(--yellow); }
.source-note { margin-top: 1.4rem; padding-top: 1rem; border-top: 1px dashed var(--line); color: #718099; font-size: .8rem; }
.memory-card { position: sticky; top: 96px; padding: 1.35rem; overflow: hidden; }
.memory-card::after { content: ""; position: absolute; width: 130px; height: 130px; right: -55px; bottom: -55px; border: 22px solid rgba(46,230,197,.08); border-radius: 50%; }
.memory-icon { width: 2.3rem; height: 2.3rem; display: grid; place-items: center; margin-bottom: 1rem; border-radius: 50%; background: var(--coral); color: #20100c; font-weight: 900; }
.memory-card h2 { margin: .5rem 0 .35rem; font-size: 1rem; }
.memory-card p:last-child { position: relative; z-index: 1; margin: 0; color: #d2dbeb; font-weight: 650; }

.quiz-section { margin-top: 1rem; padding: 1.7rem; }
.quiz-header { display: flex; justify-content: space-between; align-items: flex-start; gap: 1rem; margin-bottom: 1rem; }
.quiz-header h2 { margin: -.8rem 0 0; font-size: 1.35rem; }
.score-chip { padding: .35rem .7rem; border: 1px solid var(--line); border-radius: 99px; color: var(--muted); font-size: .82rem; white-space: nowrap; }
.question { padding: 1.25rem 0; border-top: 1px solid var(--line); }
.question:first-child { border-top: 0; }
.question h3 { display: flex; gap: .65rem; margin: 0 0 1rem; font-size: 1rem; }
.question h3 span { color: var(--coral); font: 800 .78rem ui-monospace, monospace; }
.options { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: .6rem; }
.option { display: flex; gap: .7rem; align-items: flex-start; padding: .85rem; border: 1px solid var(--line); border-radius: 11px; background: #0a1120; cursor: pointer; text-align: left; }
.option:hover:not(:disabled) { border-color: #4d607e; transform: translateY(-1px); }
.option .letter { flex: 0 0 auto; display: grid; place-items: center; width: 1.5rem; height: 1.5rem; border-radius: 50%; background: #17243a; color: #92a4bf; font: 700 .72rem ui-monospace, monospace; }
.option.correct { border-color: var(--cyan); background: rgba(46,230,197,.09); }
.option.correct .letter { background: var(--cyan); color: #04110f; }
.option.wrong { border-color: var(--danger); background: rgba(255,107,133,.08); }
.option.wrong .letter { background: var(--danger); color: #1f0710; }
.feedback { margin: .75rem 0 0; padding: .75rem .9rem; border-left: 3px solid var(--cyan); background: rgba(46,230,197,.06); color: #c8d4e5; }
.feedback.wrong { border-left-color: var(--danger); background: rgba(255,107,133,.06); }
.topic-footer { display: flex; justify-content: space-between; gap: 1rem; margin-top: 1rem; }
.nav-button { min-height: 46px; padding: .7rem 1rem; border-radius: 11px; cursor: pointer; font-weight: 760; }
.nav-button:disabled { opacity: .35; cursor: not-allowed; }
.nav-button.secondary { border: 1px solid var(--line); background: var(--panel); }
.nav-button.primary { border: 1px solid var(--cyan); background: var(--cyan); color: #04110f; }
:focus-visible { outline: 3px solid rgba(46,230,197,.65); outline-offset: 3px; }

@media (max-width: 900px) {
  .app-shell { grid-template-columns: 1fr; }
  .topbar { grid-template-columns: 1fr auto auto; }
  .brand { border-right: 0; }
  .mobile-menu { display: block; }
  .sidebar { position: fixed; z-index: 40; top: 76px; left: 0; bottom: 0; width: min(330px, 88vw); height: auto; transform: translateX(-105%); transition: transform .25s ease; box-shadow: 20px 0 70px rgba(0,0,0,.4); }
  .sidebar.open { transform: translateX(0); }
  .main { padding: 2.5rem 1.4rem 3rem; }
  .study-grid { grid-template-columns: 1fr; }
  .memory-card { position: static; }
}
@media (max-width: 620px) {
  .topbar-stats { display: none; }
  .topbar { grid-template-columns: 1fr auto; }
  .brand { padding-left: 1rem; }
  .brand span:last-child { font-size: .92rem; }
  .main { padding: 2rem 1rem; }
  .topic-count { width: 58px; height: 58px; }
  .topic-hero { gap: .8rem; }
  .topic-hero h1 { font-size: 2.35rem; }
  .topic-summary { font-size: 1rem; }
  .lesson-card, .quiz-section { padding: 1.15rem; }
  .concept-compare, .options { grid-template-columns: 1fr; }
  .quiz-header { display: block; }
  .score-chip { display: inline-block; margin-top: .9rem; }
}