:root {
  --bg: #f8fafc;
  --paper: #ffffff;
  --ink: #172033;
  --muted: #5f6b7a;
  --line: #e3e8ef;
  --purple: #7c3aed;
  --purple-dark: #5b21b6;
  --shadow: 0 18px 45px rgba(23, 32, 51, 0.08);
  --radius: 8px;
}

* {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  margin: 0;
  font-family: Arial, Helvetica, sans-serif;
  color: var(--ink);
  background: var(--bg);
  line-height: 1.65;
}

a {
  color: inherit;
}

.site-header {
  position: sticky;
  top: 0;
  z-index: 20;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
  padding: 14px clamp(18px, 4vw, 56px);
  background: rgba(255, 255, 255, 0.94);
  border-bottom: 1px solid var(--line);
  backdrop-filter: blur(12px);
}

.brand {
  font-weight: 800;
  text-decoration: none;
  color: var(--purple-dark);
  white-space: nowrap;
}

.top-nav {
  display: flex;
  flex-wrap: wrap;
  justify-content: flex-end;
  gap: 8px 18px;
  font-size: 0.95rem;
}

.top-nav a {
  text-decoration: none;
  color: var(--muted);
  font-weight: 700;
}

.top-nav a:hover {
  color: var(--purple-dark);
}

.page-shell {
  width: min(1180px, calc(100% - 32px));
  margin: 0 auto;
  padding: 44px 0 72px;
}

.study-hero,
.page-hero {
  padding: clamp(28px, 5vw, 58px) 0 30px;
}

.eyebrow,
.subject-label {
  margin: 0 0 8px;
  color: var(--purple-dark);
  font-size: 0.78rem;
  font-weight: 800;
  letter-spacing: 0;
  text-transform: uppercase;
}

h1,
h2,
h3 {
  line-height: 1.2;
  letter-spacing: 0;
}

h1 {
  position: relative;
  display: inline-block;
  margin: 0 0 18px;
  font-size: clamp(2.3rem, 6vw, 4.8rem);
}

h1::after {
  content: "";
  position: absolute;
  left: 0;
  bottom: -9px;
  width: 42%;
  min-width: 110px;
  height: 5px;
  background: var(--purple);
  border-radius: 999px;
}

.study-hero p:last-child,
.page-hero p:last-child {
  max-width: 760px;
  margin: 18px 0 0;
  color: var(--muted);
  font-size: 1.08rem;
}

.subject-grid {
  display: grid;
  grid-template-columns: repeat(5, minmax(0, 1fr));
  gap: 18px;
}

.subject-card,
.lesson-card {
  background: var(--paper);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
}

.subject-card {
  display: flex;
  min-height: 100%;
  flex-direction: column;
  overflow: hidden;
}

.subject-card__top {
  padding: 22px 20px 18px;
  border-top: 5px solid var(--accent, var(--purple));
}

.subject-card h2,
.lesson-card h2 {
  margin: 0 0 10px;
  font-size: 1.25rem;
}

.subject-card p {
  margin: 0;
  color: var(--muted);
  font-size: 0.94rem;
}

.material-links {
  display: grid;
  gap: 0;
  margin: 0;
  padding: 0 0 10px;
  list-style: none;
}

.material-links a {
  display: block;
  padding: 11px 20px;
  border-top: 1px solid var(--line);
  color: #273449;
  text-decoration: none;
  font-weight: 700;
}

.material-links a:hover {
  color: var(--purple-dark);
  background: #f3efff;
  padding-left: 25px;
}

.breadcrumbs {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  color: var(--muted);
  font-size: 0.95rem;
}

.breadcrumbs a {
  color: var(--purple-dark);
  font-weight: 800;
  text-decoration: none;
}

.content-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 18px;
}

.lesson-card {
  padding: 24px;
}

.lesson-card.wide {
  grid-column: 1 / -1;
}

.lesson-card p,
.lesson-card li,
.lesson-card td,
.lesson-card th {
  color: #334155;
}

.lesson-card ul,
.lesson-card ol {
  margin: 12px 0 0;
  padding-left: 22px;
}

.two-column-list {
  columns: 2;
  column-gap: 36px;
}

.formula-list code {
  display: inline-block;
  padding: 3px 7px;
  background: #eef2ff;
  border: 1px solid #d9e0ff;
  border-radius: 6px;
  color: #1e2a5a;
  font-family: "Courier New", monospace;
}

.table-wrap {
  overflow-x: auto;
}

table {
  width: 100%;
  border-collapse: collapse;
}

th,
td {
  padding: 10px 12px;
  border: 1px solid var(--line);
  text-align: left;
  vertical-align: top;
}

th {
  width: 34%;
  background: #f7f9fc;
}

.options {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 8px;
  margin: 8px 0;
}

.options span {
  padding: 8px 10px;
  background: #f7f9fc;
  border: 1px solid var(--line);
  border-radius: 6px;
}

.answer {
  color: #047857 !important;
  font-weight: 800;
}

.reference-note {
  border-left: 5px solid var(--purple);
}

.resource-links {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  margin-top: 14px;
}

.resource-links a {
  padding: 9px 12px;
  border: 1px solid #d8c8ff;
  border-radius: 6px;
  color: var(--purple-dark);
  background: #f7f2ff;
  text-decoration: none;
  font-weight: 800;
}

.resource-links a:hover {
  background: #ede2ff;
}

.back-to-top {
  position: fixed;
  right: 18px;
  bottom: 18px;
  width: 42px;
  height: 42px;
  border: 0;
  border-radius: 50%;
  background: var(--purple-dark);
  color: #fff;
  font-size: 1.25rem;
  cursor: pointer;
  box-shadow: 0 10px 28px rgba(91, 33, 182, 0.25);
  opacity: 0;
  transform: translateY(8px);
  pointer-events: none;
  transition: opacity 160ms ease, transform 160ms ease;
}

.back-to-top.is-visible {
  opacity: 1;
  transform: translateY(0);
  pointer-events: auto;
}

@media (max-width: 1120px) {
  .subject-grid {
    grid-template-columns: repeat(3, minmax(0, 1fr));
  }
}

@media (max-width: 760px) {
  .site-header {
    position: static;
    align-items: flex-start;
    flex-direction: column;
  }

  .top-nav {
    justify-content: flex-start;
  }

  .subject-grid,
  .content-grid {
    grid-template-columns: 1fr;
  }

  .two-column-list {
    columns: 1;
  }

  .options {
    grid-template-columns: 1fr;
  }

  .lesson-card {
    padding: 20px;
  }
}

@media (max-width: 520px) {
  .page-shell {
    width: min(100% - 22px, 1180px);
    padding-top: 24px;
  }

  h1 {
    font-size: 2.35rem;
  }

  .subject-card__top,
  .material-links a {
    padding-left: 16px;
    padding-right: 16px;
  }
}
.board-materials {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: clamp(28px, 8vw, 86px);
  margin-top: 42px;
  padding: clamp(26px, 5vw, 42px);
  color: #f8fafc;
  background: #1c213d;
  border-radius: var(--radius);
  box-shadow: var(--shadow);
}

.board-column h2 {
  position: relative;
  margin: 0 0 22px;
  padding-bottom: 14px;
  font-size: 1.15rem;
  color: #ffffff;
}

.board-column h2::after {
  content: "";
  position: absolute;
  left: 0;
  bottom: 0;
  width: 52px;
  height: 3px;
  background: #f59e0b;
  border-radius: 999px;
}

.board-column ul {
  display: grid;
  gap: 11px;
  margin: 0;
  padding: 0;
  list-style: none;
}

.board-column a {
  color: #dbe4f0;
  text-decoration: none;
  font-weight: 700;
}

.board-column a:hover {
  color: #ffffff;
  text-decoration: underline;
  text-decoration-color: #f59e0b;
  text-underline-offset: 5px;
}

@media (max-width: 760px) {
  .board-materials {
    grid-template-columns: 1fr;
  }
}