:root {
  --color-bg: #f4f7fb;
  --color-surface: rgba(255, 255, 255, 0.84);
  --color-surface-strong: #ffffff;
  --color-surface-muted: #eef4ff;
  --color-text: #132238;
  --color-text-soft: #526277;
  --color-border: rgba(19, 34, 56, 0.1);
  --color-shadow: rgba(15, 23, 42, 0.08);
  --color-accent: #1f6feb;
  --color-accent-strong: #164fce;
  --color-accent-soft: #dbe9ff;
  --color-definition: #eff8ff;
  --color-theorem: #eefcf4;
  --color-proposition: #f5f1ff;
  --color-corollary: #fff8e8;
  --color-example: #fff1eb;
  --color-proof: #f6f8fb;
  --font-sans: "Segoe UI", "SF Pro Display", "Helvetica Neue", Arial, sans-serif;
  --font-serif: "Iowan Old Style", "Palatino Linotype", "Book Antiqua", Georgia, serif;
  --space-1: 0.25rem;
  --space-2: 0.5rem;
  --space-3: 0.75rem;
  --space-4: 1rem;
  --space-5: 1.5rem;
  --space-6: 2rem;
  --space-7: 3rem;
  --space-8: 4rem;
  --radius-sm: 0.75rem;
  --radius-md: 1rem;
  --radius-lg: 1.5rem;
  --shadow-sm: 0 10px 30px var(--color-shadow);
  --shadow-lg: 0 24px 60px rgba(15, 23, 42, 0.12);
  --container-width: 1200px;
  --transition: 180ms ease;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body,
h1,
h2,
h3,
p,
figure,
ol,
ul {
  margin: 0;
}

body {
  min-height: 100vh;
  font-family: var(--font-sans);
  line-height: 1.7;
  color: var(--color-text);
  background:
    radial-gradient(circle at top left, rgba(31, 111, 235, 0.16), transparent 28%),
    radial-gradient(circle at top right, rgba(120, 160, 255, 0.12), transparent 24%),
    linear-gradient(180deg, #f9fbff 0%, var(--color-bg) 45%, #eef3fb 100%);
}

img {
  display: block;
  max-width: 100%;
}

a {
  color: inherit;
  text-decoration: none;
}

button {
  font: inherit;
}

.skip-link {
  position: absolute;
  left: var(--space-4);
  top: -3rem;
  z-index: 100;
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-sm);
  background: var(--color-text);
  color: #fff;
  transition: top var(--transition);
}

.skip-link:focus {
  top: var(--space-4);
}

.container {
  width: min(calc(100% - 2rem), var(--container-width));
  margin-inline: auto;
}

.site-header {
  padding: var(--space-5) 0 var(--space-4);
}

.site-header__inner {
  display: flex;
  flex-direction: column;
  gap: var(--space-4);
  align-items: flex-start;
}

.brand__eyebrow,
.section-kicker,
.lesson-card__label {
  text-transform: uppercase;
  letter-spacing: 0.12em;
  font-size: 0.76rem;
  font-weight: 700;
}

.brand__eyebrow,
.section-kicker {
  color: var(--color-accent);
}

.brand__title {
  max-width: 16ch;
  font-family: var(--font-serif);
  font-size: clamp(2rem, 6vw, 4.25rem);
  line-height: 1.05;
}

.brand__subtitle {
  max-width: 64ch;
  color: var(--color-text-soft);
}

.author-chip {
  display: inline-flex;
  flex-direction: column;
  gap: var(--space-1);
  padding: var(--space-3) var(--space-4);
  border: 1px solid var(--color-border);
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.72);
  box-shadow: var(--shadow-sm);
  cursor: pointer;
  transition:
    transform var(--transition),
    box-shadow var(--transition),
    border-color var(--transition);
}

.author-chip:hover,
.author-chip:focus-visible {
  transform: translateY(-2px);
  border-color: rgba(31, 111, 235, 0.28);
  box-shadow: 0 18px 36px rgba(15, 23, 42, 0.12);
  outline: none;
}

.author-chip__label {
  font-size: 0.72rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.12em;
  color: var(--color-text-soft);
}

.author-chip__value {
  font-weight: 700;
}

.hero {
  padding: var(--space-4) 0 var(--space-7);
}

.hero__grid {
  display: grid;
  gap: var(--space-6);
}

.hero__content,
.hero-card,
.lesson,
.sidebar__panel {
  border: 1px solid var(--color-border);
  background: var(--color-surface);
  backdrop-filter: blur(18px);
  box-shadow: var(--shadow-sm);
}

.hero__content {
  padding: var(--space-6);
  border-radius: var(--radius-lg);
}

.hero__title {
  max-width: 14ch;
  margin-top: var(--space-3);
  font-family: var(--font-serif);
  font-size: clamp(1.8rem, 5vw, 3rem);
  line-height: 1.1;
}

.hero__text {
  max-width: 58ch;
  margin-top: var(--space-4);
  color: var(--color-text-soft);
}

.hero__highlights {
  display: flex;
  flex-wrap: wrap;
  gap: var(--space-2);
  margin-top: var(--space-5);
}

.pill {
  padding: 0.7rem 0.95rem;
  border-radius: 999px;
  background: var(--color-accent-soft);
  color: var(--color-accent-strong);
  font-size: 0.92rem;
  font-weight: 600;
}

.hero__quote {
  min-height: 3.5rem;
  margin-top: var(--space-5);
  padding-top: var(--space-4);
  border-top: 1px solid var(--color-border);
  color: var(--color-text-soft);
}

.hero-card {
  overflow: hidden;
  border-radius: var(--radius-lg);
}

.hero-card__image {
  aspect-ratio: 4 / 3;
  object-fit: cover;
}

.hero-card__caption {
  padding: var(--space-4);
  color: var(--color-text-soft);
}

.content-section {
  padding-bottom: var(--space-8);
}

.content-grid {
  display: grid;
  gap: var(--space-5);
}

.lesson {
  padding: var(--space-6);
  border-radius: var(--radius-lg);
}

.lesson__header {
  margin-bottom: var(--space-6);
}

.lesson__title,
.sidebar__title {
  margin-top: var(--space-2);
  font-family: var(--font-serif);
  line-height: 1.15;
}

.lesson__title {
  font-size: clamp(1.6rem, 4vw, 2.25rem);
}

.lesson-section + .lesson-section {
  margin-top: var(--space-7);
}

.lesson-section__heading {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  margin-bottom: var(--space-4);
}

.lesson-section__heading h3 {
  font-size: clamp(1.15rem, 3vw, 1.5rem);
}

.lesson-section__index {
  display: inline-grid;
  place-items: center;
  width: 2.5rem;
  height: 2.5rem;
  border-radius: 999px;
  background: var(--color-text);
  color: #fff;
  font-weight: 700;
}

.lesson-section__intro {
  margin-bottom: var(--space-4);
  color: var(--color-text-soft);
}

.lesson-card {
  position: relative;
  padding: var(--space-5);
  border: 1px solid transparent;
  border-radius: var(--radius-md);
  transition:
    transform var(--transition),
    box-shadow var(--transition),
    border-color var(--transition);
}

.lesson-card + .lesson-card {
  margin-top: var(--space-4);
}

.lesson-card::before {
  content: "";
  position: absolute;
  inset: 0 auto 0 0;
  width: 0.35rem;
  border-radius: inherit;
  background: currentColor;
  opacity: 0.18;
}

.lesson-card:hover {
  transform: translateY(-3px);
  box-shadow: var(--shadow-lg);
}

.lesson-card__label {
  margin-bottom: var(--space-3);
}

.lesson-card__body {
  color: var(--color-text);
}

.lesson-card__body a {
  color: var(--color-accent-strong);
  text-decoration: underline;
  text-underline-offset: 0.15em;
}

.lesson-card--definition {
  background: var(--color-definition);
  color: #0f5f9c;
}

.lesson-card--theorem {
  background: var(--color-theorem);
  color: #1f7a46;
}

.lesson-card--proposition {
  background: var(--color-proposition);
  color: #6545b6;
}

.lesson-card--corollary {
  background: var(--color-corollary);
  color: #9a6f1e;
}

.lesson-card--example {
  background: var(--color-example);
  color: #bf5a22;
}

.lesson-card--proof {
  background: var(--color-proof);
  color: #4c5d73;
}

.sidebar {
  display: grid;
  gap: var(--space-4);
  align-self: start;
}

.sidebar__panel {
  padding: var(--space-5);
  border-radius: var(--radius-lg);
}

.sidebar__panel--muted {
  background: var(--color-surface-muted);
}

.summary-list,
.check-list {
  padding-left: 1.25rem;
  margin-top: var(--space-4);
  color: var(--color-text-soft);
}

.summary-list li + li,
.check-list li + li {
  margin-top: var(--space-3);
}

.summary-list a {
  color: var(--color-text);
  transition: color var(--transition);
}

.summary-list a:hover,
.summary-list a:focus-visible {
  color: var(--color-accent-strong);
  outline: none;
}

@media (min-width: 48rem) {
  .site-header__inner {
    flex-direction: row;
    justify-content: space-between;
    align-items: flex-start;
  }

  .hero__grid {
    grid-template-columns: minmax(0, 1.35fr) minmax(18rem, 0.9fr);
    align-items: stretch;
  }
}

@media (min-width: 64rem) {
  .content-grid {
    grid-template-columns: minmax(0, 1.65fr) minmax(18rem, 0.72fr);
    align-items: start;
  }

  .sidebar {
    position: sticky;
    top: var(--space-4);
  }
}

@media (prefers-reduced-motion: reduce) {
  html {
    scroll-behavior: auto;
  }

  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
  }
}