/* ===== CSS Custom Properties (Color Scheme) ===== */
:root {
  /* Light theme (default) */
  --color-bg-primary: #ffffff;
  --color-bg-secondary: #f5f7fa;
  --color-bg-tertiary: #e8ecf1;
  --color-text-primary: #1a1a2e;
  --color-text-secondary: #4a4a6a;
  --color-text-muted: #8a8aa0;
  --color-accent: #4361ee;
  --color-accent-hover: #3a56d4;
  --color-accent-light: #eef1ff;
  --color-border: #d4d8e0;
  --color-card-bg: #ffffff;
  --color-card-shadow: rgba(0, 0, 0, 0.08);
  --color-header-bg: #ffffff;
  --color-header-border: #e8ecf1;
  --color-footer-bg: #1a1a2e;
  --color-footer-text: #c4c4d4;
  --color-footer-heading: #ffffff;
  --color-btn-primary-bg: #4361ee;
  --color-btn-primary-text: #ffffff;
  --color-btn-primary-hover: #3a56d4;
  --color-success: #2ec4b6;
  --color-warning: #ff9f1c;
  --color-danger: #e71d36;
  --color-checked: #2ec4b6;
  --color-check-bg: #ffffff;
  --font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
  --font-family-heading: 'Inter', system-ui, -apple-system, 'Segoe UI', Roboto, sans-serif;
  --font-size-xs: 0.75rem;
  --font-size-sm: 0.875rem;
  --font-size-base: 1rem;
  --font-size-lg: 1.125rem;
  --font-size-xl: 1.25rem;
  --font-size-2xl: 1.5rem;
  --font-size-3xl: 2rem;
  --font-size-4xl: 2.5rem;
  --font-weight-normal: 400;
  --font-weight-medium: 500;
  --font-weight-semibold: 600;
  --font-weight-bold: 700;
  --line-height-tight: 1.2;
  --line-height-normal: 1.5;
  --line-height-relaxed: 1.75;
  --spacing-xs: 0.25rem;
  --spacing-sm: 0.5rem;
  --spacing-md: 1rem;
  --spacing-lg: 1.5rem;
  --spacing-xl: 2rem;
  --spacing-2xl: 3rem;
  --spacing-3xl: 4rem;
  --spacing-4xl: 6rem;
  --border-radius-sm: 4px;
  --border-radius-md: 8px;
  --border-radius-lg: 12px;
  --border-radius-xl: 16px;
  --border-radius-full: 50%;
  --shadow-sm: 0 1px 3px var(--color-card-shadow);
  --shadow-md: 0 4px 12px var(--color-card-shadow);
  --shadow-lg: 0 8px 24px var(--color-card-shadow);
  --container-max-width: 1200px;
  --container-padding: var(--spacing-lg);
  --header-height: 64px;
  --transition-fast: 0.2s ease;
  --transition-normal: 0.3s ease;
  --transition-slow: 0.4s ease;
}

/* ===== Reset Styles ===== */
*,
*::before,
*::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
  -webkit-text-size-adjust: 100%;
  -moz-text-size-adjust: 100%;
  text-size-adjust: 100%;
}

body {
  font-family: var(--font-family);
  font-size: var(--font-size-base);
  line-height: var(--line-height-normal);
  color: var(--color-text-primary);
  background-color: var(--color-bg-primary);
  min-height: 100vh;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

img,
svg,
video {
  display: block;
  max-width: 100%;
  height: auto;
}

a {
  color: var(--color-accent);
  text-decoration: none;
  transition: color var(--transition-fast);
}

a:hover {
  color: var(--color-accent-hover);
}

a:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 2px;
  border-radius: var(--border-radius-sm);
}

ul,
ol {
  list-style: none;
}

button {
  font-family: inherit;
  font-size: inherit;
  cursor: pointer;
  border: none;
  background: none;
  color: inherit;
}

button:focus-visible {
  outline: 2px solid var(--color-accent);
  outline-offset: 2px;
}

input,
textarea,
select {
  font-family: inherit;
  font-size: inherit;
  color: inherit;
}

h1, h2, h3, h4, h5, h6 {
  font-family: var(--font-family-heading);
  font-weight: var(--font-weight-bold);
  line-height: var(--line-height-tight);
  color: var(--color-text-primary);
}

h1 {
  font-size: var(--font-size-4xl);
  margin-bottom: var(--spacing-md);
}

h2 {
  font-size: var(--font-size-3xl);
  margin-bottom: var(--spacing-md);
}

h3 {
  font-size: var(--font-size-2xl);
  margin-bottom: var(--spacing-sm);
}

h4 {
  font-size: var(--font-size-xl);
  margin-bottom: var(--spacing-sm);
}

p {
  margin-bottom: var(--spacing-md);
  color: var(--color-text-secondary);
}

/* ===== Utility Classes ===== */
.container {
  max-width: var(--container-max-width);
  margin: 0 auto;
  padding: 0 var(--container-padding);
}

.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;
}

.section {
  padding: var(--spacing-4xl) 0;
}

.section__title {
  text-align: center;
  margin-bottom: var(--spacing-sm);
}

.section__subtitle {
  text-align: center;
  color: var(--color-text-muted);
  font-size: var(--font-size-lg);
  margin-bottom: var(--spacing-2xl);
}

/* ===== Button Styles ===== */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--spacing-sm);
  padding: var(--spacing-sm) var(--spacing-lg);
  border-radius: var(--border-radius-md);
  font-weight: var(--font-weight-medium);
  font-size: var(--font-size-sm);
  transition: background-color var(--transition-fast), color var(--transition-fast), transform var(--transition-fast);
  cursor: pointer;
  text-decoration: none;
}

.btn--primary {
  background-color: var(--color-btn-primary-bg);
  color: var(--color-btn-primary-text);
}

.btn--primary:hover {
  background-color: var(--color-btn-primary-hover);
  color: var(--color-btn-primary-text);
  transform: translateY(-1px);
}

.btn--outline {
  border: 2px solid var(--color-accent);
  color: var(--color-accent);
  background-color: transparent;
}

.btn--outline:hover {
  background-color: var(--color-accent);
  color: #ffffff;
}

.btn--lg {
  padding: var(--spacing-md) var(--spacing-xl);
  font-size: var(--font-size-base);
}

/* ===== Accordion Styles ===== */
.accordion {
  border: 1px solid var(--color-border);
  border-radius: var(--border-radius-md);
  overflow: hidden;
}

.accordion__item {
  border-bottom: 1px solid var(--color-border);
}

.accordion__item:last-child {
  border-bottom: none;
}

.accordion__header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  width: 100%;
  padding: var(--spacing-md) var(--spacing-lg);
  background-color: var(--color-bg-primary);
  color: var(--color-text-primary);
  font-weight: var(--font-weight-medium);
  font-size: var(--font-size-base);
  text-align: left;
  cursor: pointer;
  transition: background-color var(--transition-fast);
}

.accordion__header:hover {
  background-color: var(--color-bg-secondary);
}

.accordion__icon {
  width: 20px;
  height: 20px;
  transition: transform var(--transition-normal);
  flex-shrink: 0;
}

.accordion__item.active .accordion__icon {
  transform: rotate(180deg);
}

.accordion__body {
  max-height: 0;
  overflow: hidden;
  transition: max-height var(--transition-slow), padding var(--transition-slow);
}

.accordion__item.active .accordion__body {
  max-height: 500px;
}

.accordion__content {
  padding: 0 var(--spacing-lg) var(--spacing-lg);
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
  line-height: var(--line-height-relaxed);
}

/* ===== Checklist Styles ===== */
.checklist {
  display: flex;
  flex-direction: column;
  gap: var(--spacing-sm);
}

.checklist__item {
  display: flex;
  align-items: flex-start;
  gap: var(--spacing-sm);
  padding: var(--spacing-sm) var(--spacing-md);
  border-radius: var(--border-radius-md);
  cursor: pointer;
  transition: background-color var(--transition-fast);
}

.checklist__item:hover {
  background-color: var(--color-bg-secondary);
}

.checklist__checkbox {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 22px;
  height: 22px;
  border: 2px solid var(--color-border);
  border-radius: var(--border-radius-sm);
  flex-shrink: 0;
  margin-top: 2px;
  transition: background-color var(--transition-fast), border-color var(--transition-fast);
}

.checklist__item.checked .checklist__checkbox {
  background-color: var(--color-checked);
  border-color: var(--color-checked);
}

.checklist__check-icon {
  width: 14px;
  height: 14px;
  fill: #ffffff;
  display: none;
}

.checklist__item.checked .checklist__check-icon {
  display: block;
}

.checklist__label {
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
  line-height: var(--line-height-relaxed);
  user-select: none;
}

.checklist__item.checked .checklist__label {
  text-decoration: line-through;
  color: var(--color-text-muted);
}

/* ===== Hero Section ===== */
.hero {
  padding: var(--spacing-4xl) 0;
  background: linear-gradient(135deg, var(--color-accent-light) 0%, var(--color-bg-secondary) 100%);
}

.hero__inner {
  display: flex;
  align-items: center;
  gap: var(--spacing-3xl);
  max-width: var(--container-max-width);
  margin: 0 auto;
  padding: 0 var(--container-padding);
}

.hero__content {
  flex: 1;
}

.hero__title {
  font-size: var(--font-size-4xl);
  margin-bottom: var(--spacing-md);
  color: var(--color-text-primary);
}

.hero__subtitle {
  font-size: var(--font-size-lg);
  color: var(--color-text-secondary);
  margin-bottom: var(--spacing-xl);
  line-height: var(--line-height-relaxed);
  max-width: 600px;
}

.hero__actions {
  display: flex;
  gap: var(--spacing-md);
  flex-wrap: wrap;
}

.hero__image {
  flex: 1;
  max-width: 500px;
  display: flex;
  align-items: center;
  justify-content: center;
}

.hero__image-placeholder {
  width: 100%;
  aspect-ratio: 4 / 3;
  background: linear-gradient(135deg, var(--color-accent), var(--color-accent-hover));
  border-radius: var(--border-radius-xl);
  display: flex;
  align-items: center;
  justify-content: center;
  color: #ffffff;
  font-size: var(--font-size-4xl);
  font-weight: var(--font-weight-bold);
  box-shadow: var(--shadow-lg);
}

/* ===== Page Header ===== */
.page-header {
  padding: var(--spacing-3xl) 0 var(--spacing-xl);
  background-color: var(--color-bg-secondary);
  text-align: center;
}

.page-header__title {
  font-size: var(--font-size-3xl);
  margin-bottom: var(--spacing-sm);
}

.page-header__subtitle {
  font-size: var(--font-size-lg);
  color: var(--color-text-secondary);
  max-width: 700px;
  margin: 0 auto;
}

/* ===== Category Content Layout ===== */
.category-content {
  max-width: var(--container-max-width);
  margin: 0 auto;
  padding: 0 var(--container-padding);
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--spacing-3xl);
  padding-top: var(--spacing-2xl);
  padding-bottom: var(--spacing-2xl);
}

.category-content__full {
  max-width: var(--container-max-width);
  margin: 0 auto;
  padding: 0 var(--container-padding);
  padding-top: var(--spacing-2xl);
  padding-bottom: var(--spacing-2xl);
}

/* ===== Features List ===== */
.features-list {
  display: flex;
  flex-direction: column;
  gap: var(--spacing-lg);
}

.feature-item {
  display: flex;
  gap: var(--spacing-md);
  padding: var(--spacing-lg);
  background-color: var(--color-bg-secondary);
  border-radius: var(--border-radius-md);
  border-left: 3px solid var(--color-accent);
}

.feature-item__icon {
  width: 40px;
  height: 40px;
  flex-shrink: 0;
  background-color: var(--color-accent-light);
  border-radius: var(--border-radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-accent);
  font-size: var(--font-size-xl);
}

.feature-item__content h4 {
  font-size: var(--font-size-base);
  margin-bottom: var(--spacing-xs);
}

.feature-item__content p {
  font-size: var(--font-size-sm);
  margin-bottom: 0;
}

/* ===== Illustration Placeholder ===== */
.illustration {
  width: 100%;
  aspect-ratio: 16 / 10;
  background: linear-gradient(135deg, var(--color-accent-light), var(--color-bg-tertiary));
  border-radius: var(--border-radius-lg);
  display: flex;
  align-items: center;
  justify-content: center;
  color: var(--color-accent);
  font-size: var(--font-size-3xl);
  font-weight: var(--font-weight-bold);
  border: 2px dashed var(--color-border);
}

.illustration--sm {
  aspect-ratio: 16 / 9;
  font-size: var(--font-size-xl);
}

/* ===== 404 Page ===== */
.error-page {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 70vh;
  text-align: center;
  padding: var(--spacing-2xl) var(--container-padding);
}

.error-page__inner {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: var(--spacing-lg);
  max-width: 560px;
}

.error-page__illustration {
  width: 200px;
  height: 160px;
  color: var(--color-accent);
  opacity: 0.8;
  margin-bottom: var(--spacing-sm);
}

.error-page__illustration svg {
  width: 100%;
  height: 100%;
}

.error-page__code {
  font-size: 8rem;
  font-weight: var(--font-weight-bold);
  color: var(--color-accent);
  line-height: 1;
  margin-bottom: var(--spacing-md);
  display: flex;
  align-items: center;
  gap: var(--spacing-sm);
}

.error-page__code-emoji {
  font-size: 5rem;
  display: inline-block;
}

.error-page__title {
  font-size: var(--font-size-2xl);
  margin-bottom: var(--spacing-sm);
}

.error-page__text {
  font-size: var(--font-size-base);
  color: var(--color-text-secondary);
  margin-bottom: var(--spacing-lg);
  max-width: 480px;
  margin-left: auto;
  margin-right: auto;
  line-height: var(--line-height-relaxed);
}

.error-page__actions {
  display: flex;
  gap: var(--spacing-md);
  flex-wrap: wrap;
  justify-content: center;
}

/* ===== Breadcrumbs ===== */
.breadcrumbs {
  display: flex;
  align-items: center;
  gap: var(--spacing-xs);
  padding: var(--spacing-md) 0;
  font-size: var(--font-size-sm);
  color: var(--color-text-muted);
  max-width: var(--container-max-width);
  margin: 0 auto;
  padding-left: var(--container-padding);
  padding-right: var(--container-padding);
}

.breadcrumbs a {
  color: var(--color-text-secondary);
}

.breadcrumbs a:hover {
  color: var(--color-accent);
}

.breadcrumbs__separator {
  margin: 0 var(--spacing-xs);
}

/* ===== Responsive Styles (Mobile-first) ===== */

/* Base: 320px+ (mobile defaults) */
:root {
  --container-padding: var(--spacing-md);
}

h1 {
  font-size: var(--font-size-3xl);
}

h2 {
  font-size: var(--font-size-2xl);
}

.hero {
  padding: var(--spacing-2xl) 0;
}

.hero__inner {
  flex-direction: column;
  text-align: center;
}

.hero__content {
  display: flex;
  flex-direction: column;
  align-items: center;
}

.hero__title {
  font-size: var(--font-size-3xl);
}

.hero__subtitle {
  max-width: 100%;
}

.hero__actions {
  flex-direction: column;
  align-items: stretch;
  justify-content: center;
}

.hero__image {
  display: none;
}

.section {
  padding: var(--spacing-2xl) 0;
}

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

.error-page__code {
  font-size: 4.5rem;
}

.error-page__code-emoji {
  font-size: 3rem;
}

.error-page__illustration {
  width: 160px;
  height: 128px;
}

.btn--lg {
  text-align: center;
}

/* Small screens: 480px+ */
@media (min-width: 480px) {
  .hero__actions {
    flex-direction: row;
    align-items: center;
  }

  .btn--lg {
    text-align: left;
  }
}

/* Tablet: 768px+ */
@media (min-width: 768px) {
  :root {
    --container-padding: var(--spacing-lg);
  }

  h1 {
    font-size: var(--font-size-4xl);
  }

  h2 {
    font-size: var(--font-size-3xl);
  }

  .hero {
    padding: var(--spacing-4xl) 0;
  }

  .hero__inner {
    flex-direction: column;
    text-align: center;
  }

  .hero__title {
    font-size: var(--font-size-4xl);
  }

  .hero__image {
    display: flex;
  }

  .section {
    padding: var(--spacing-4xl) 0;
  }

  .error-page__code {
    font-size: 7rem;
  }

  .error-page__code-emoji {
    font-size: 4.5rem;
  }

  .error-page__illustration {
    width: 200px;
    height: 160px;
  }
}

/* Desktop: 1024px+ */
@media (min-width: 1024px) {
  .hero__inner {
    flex-direction: row;
    text-align: left;
  }

  .hero__content {
    align-items: flex-start;
  }

  .hero__subtitle {
    max-width: 600px;
  }

  .hero__actions {
    justify-content: flex-start;
  }

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