/* Basic CSS Reset */
*, *::before, *::after {
  box-sizing: border-box;
}

body, h1, h2, h3, p, ul, li, figure, figcaption, blockquote, dl, dd {
  margin: 0;
}

ul {
  padding: 0;
  list-style: none;
}

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

/* Custom Properties */
:root {
  /* Colors */
  --color-paper: #fdfbf7;
  --color-ink: #1a237e; /* Ink Blue */
  --color-marker: #ff9800; /* Marker Orange */
  --color-line: #e0e0e0;
  --color-marker-light: rgba(255, 152, 0, 0.2);

  /* Fonts */
  --font-body: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol';
  --font-heading: 'Comic Sans MS', 'Comic Sans', 'Chalkboard SE', 'Marker Felt', 'Permanent Marker', cursive, sans-serif;

  /* Spacing */
  --space-xs: 0.5rem;
  --space-sm: 1rem;
  --space-md: 1.5rem;
  --space-lg: 2rem;
  --space-xl: 3rem;

  /* Notebook Lines */
  --line-height: 1.6;
  --bg-lines: linear-gradient(transparent 95%, var(--color-line) 100%);
  --bg-size: 100% calc(1rem * var(--line-height));
}

/* Dark Mode Support */
@media (prefers-color-scheme: dark) {
  :root {
    --color-paper: #121212;
    --color-ink: #90caf9;
    --color-marker: #ffb74d;
    --color-line: #333333;
    --color-marker-light: rgba(255, 183, 77, 0.2);
  }
}

/* Base Styles */
body {
  font-family: var(--font-body);
  color: var(--color-ink);
  background-color: var(--color-paper);
  background-image: var(--bg-lines);
  background-size: var(--bg-size);
  line-height: var(--line-height);
  font-size: 1rem;
  padding: 0 var(--space-sm);
  /* Offset background so text sits on the lines */
  background-position: 0 0.2rem;
}

/* Focus States for Accessibility */
:focus-visible {
  outline: 3px solid var(--color-marker);
  outline-offset: 2px;
  border-radius: 2px;
}

/* Accessibility: Reduced Motion */
@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    animation-duration: 0.01ms !important;
    animation-iteration-count: 1 !important;
    transition-duration: 0.01ms !important;
    scroll-behavior: auto !important;
  }
}

/* Layout */
.container {
  max-width: 800px;
  margin: 0 auto;
  padding: var(--space-xl) 0;
  background-color: var(--color-paper); /* Ensure text covers lines in some contexts if needed, though usually we let it bleed */
}

/* Typography */
h1, h2, h3, h4 {
  font-family: var(--font-heading);
  line-height: 1.2;
  margin-bottom: var(--space-md);
  margin-top: var(--space-lg);
  color: var(--color-ink);
}

h1 {
  font-size: clamp(2rem, 5vw, 3.5rem);
  text-align: center;
  position: relative;
  display: inline-block;
  margin: 0 auto;
}

h1::after {
  content: '';
  position: absolute;
  bottom: -5px;
  left: -5%;
  width: 110%;
  height: 8px;
  background-color: var(--color-marker);
  border-radius: 4px;
  z-index: -1;
  opacity: 0.8;
  transform: rotate(-1deg);
}

h2 {
  font-size: clamp(1.5rem, 4vw, 2.5rem);
  border-bottom: 2px dashed var(--color-marker);
  padding-bottom: var(--space-xs);
  display: inline-block;
}

h3 {
  font-size: 1.25rem;
}

p {
  margin-bottom: var(--space-md);
}

a {
  color: var(--color-ink);
  text-decoration-color: var(--color-marker);
  text-decoration-thickness: 2px;
  text-underline-offset: 2px;
}

a:hover, a:focus {
  background-color: var(--color-marker-light);
  color: var(--color-ink);
}

/* Header & Footer */
header {
  text-align: center;
  margin-bottom: var(--space-xl);
}

.main-nav {
  margin-bottom: var(--space-xl);
}

.main-nav ul {
  display: flex;
  justify-content: center;
  gap: var(--space-md);
  flex-wrap: wrap;
}

.main-nav a {
  font-family: var(--font-heading);
  font-size: 1.1rem;
  text-decoration: none;
  color: var(--color-ink);
  padding: var(--space-xs) var(--space-sm);
  border: 2px solid transparent;
  border-radius: 4px;
  transition: all 0.2s ease;
}

.main-nav a:hover, .main-nav a:focus {
  border-color: var(--color-marker);
  background-color: var(--color-marker-light);
}

.hero {
  padding: var(--space-md) 0 var(--space-xl) 0;
  display: flex;
  flex-direction: column;
  align-items: center;
}

.hero-content {
  text-align: center;
  display: flex;
  flex-direction: column;
  align-items: center;
}

.intro {
  font-size: 1.1rem;
  text-align: center;
  margin-bottom: var(--space-lg);
  max-width: 600px;
  margin-left: auto;
  margin-right: auto;
  background-color: var(--color-marker-light);
  padding: var(--space-md);
  border-radius: 8px;
  border: 1px solid var(--color-marker);
}

.btn-primary {
  display: inline-block;
  background-color: var(--color-ink);
  color: var(--color-paper);
  font-family: var(--font-heading);
  font-size: 1.2rem;
  padding: var(--space-sm) var(--space-lg);
  border-radius: 4px;
  text-decoration: none;
  border: 2px solid var(--color-ink);
  transition: transform 0.1s ease, box-shadow 0.1s ease;
  box-shadow: 4px 4px 0 var(--color-marker);
}

.btn-primary:hover, .btn-primary:focus {
  transform: translate(2px, 2px);
  box-shadow: 2px 2px 0 var(--color-marker);
  color: var(--color-paper);
  background-color: var(--color-ink);
}

footer {
  margin-top: var(--space-xl);
  padding-top: var(--space-lg);
  border-top: 2px solid var(--color-line);
  text-align: center;
  font-size: 0.9rem;
}

footer ul {
  display: flex;
  justify-content: center;
  gap: var(--space-md);
  margin-top: var(--space-sm);
}

/* Course Progress */
.progress-container {
  margin-bottom: var(--space-lg);
  padding: var(--space-md);
  border: 2px solid var(--color-ink);
  border-radius: 8px;
  background-color: var(--color-paper);
}

.progress-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: var(--space-xs);
  font-family: var(--font-heading);
  font-size: 1.2rem;
}

.progress-bar-bg {
  width: 100%;
  height: 20px;
  background-color: var(--color-line);
  border-radius: 10px;
  overflow: hidden;
  border: 1px solid var(--color-ink);
}

.progress-bar-fill {
  height: 100%;
  background-color: var(--color-marker);
  width: 0%;
  transition: width 0.3s ease;
}

.completion-badge {
  margin-top: var(--space-md);
  padding: var(--space-md);
  background-color: var(--color-marker-light);
  border: 2px solid var(--color-marker);
  border-radius: 8px;
  text-align: center;
  animation: popIn 0.5s cubic-bezier(0.175, 0.885, 0.32, 1.275) forwards;
}

@keyframes popIn {
  0% { transform: scale(0.9); opacity: 0; }
  100% { transform: scale(1); opacity: 1; }
}

.btn-reset {
  margin-top: var(--space-sm);
  background-color: var(--color-paper);
  color: var(--color-ink);
  font-family: var(--font-heading);
  font-size: 1rem;
  padding: var(--space-xs) var(--space-md);
  border-radius: 4px;
  cursor: pointer;
  border: 2px solid var(--color-ink);
  transition: background-color 0.2s ease, transform 0.1s ease;
}

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

/* Lesson Cards */
.lesson {
  margin-bottom: var(--space-lg);
  border: 2px solid var(--color-ink);
  border-radius: 8px;
  background-color: var(--color-paper);
  box-shadow: 4px 4px 0 var(--color-marker-light);
  transition: transform 0.2s ease, box-shadow 0.2s ease;
  overflow: hidden;
}

.lesson:focus-within {
  box-shadow: 6px 6px 0 var(--color-marker);
  transform: translateY(-2px);
}

.lesson-header {
  padding: var(--space-sm) var(--space-md);
  display: flex;
  justify-content: space-between;
  align-items: center;
  cursor: pointer;
  background-color: var(--color-paper);
  position: relative;
  z-index: 2;
  transition: background-color 0.2s ease;
}

.lesson-header-left {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
}

.lesson-header h2 {
  margin: 0;
  border: none;
  font-size: 1.25rem;
}

.lesson-badge {
  display: none;
  color: var(--color-marker);
  font-family: var(--font-heading);
  font-size: 1.25rem;
  font-weight: bold;
}

.lesson.is-completed .lesson-badge {
  display: inline-block;
}

.lesson-toggle {
  background: none;
  border: none;
  font-family: var(--font-heading);
  color: var(--color-ink);
  font-size: 1.25rem;
  cursor: pointer;
  padding: var(--space-xs);
  transition: transform 0.3s ease;
}

/* Smooth expansion using grid */
.lesson-content-wrapper {
  display: grid;
  grid-template-rows: 0fr;
  transition: grid-template-rows 0.3s ease;
}

.lesson-content {
  overflow: hidden;
}

.lesson-content-inner {
  padding: var(--space-sm) var(--space-md) var(--space-md) var(--space-md);
  border-top: 2px solid var(--color-line);
}

.lesson.is-open .lesson-content-wrapper {
  grid-template-rows: 1fr;
}

.lesson.is-open .lesson-toggle {
  transform: rotate(180deg);
}

.lesson.is-open .lesson-header {
  background-color: var(--color-marker-light);
}

.takeaway {
  background-color: var(--color-marker-light);
  padding: var(--space-sm);
  border-left: 4px solid var(--color-marker);
  margin: var(--space-md) 0;
  font-weight: 500;
}

.takeaway strong {
  font-family: var(--font-heading);
}

.exercise {
  margin-top: var(--space-md);
  padding: var(--space-md);
  border: 1px dashed var(--color-ink);
  border-radius: 4px;
}

.exercise h3 {
  margin-top: 0;
}

/* Checkbox Styling */
.checkbox-container {
  display: flex;
  align-items: center;
  gap: var(--space-sm);
  margin-top: var(--space-md);
  font-weight: bold;
  cursor: pointer;
}

.checkbox-container input {
  opacity: 0;
  position: absolute;
}

.checkmark {
  width: 24px;
  height: 24px;
  border: 2px solid var(--color-ink);
  border-radius: 4px;
  display: flex;
  align-items: center;
  justify-content: center;
  background-color: var(--color-paper);
}

.checkbox-container input:checked + .checkmark::after {
  content: '✓';
  color: var(--color-marker);
  font-family: var(--font-heading);
  font-size: 20px;
}

.checkbox-container input:focus-visible + .checkmark {
  outline: 3px solid var(--color-marker);
  outline-offset: 2px;
}

/* Pages (Impressum, Datenschutz, 404) */
.page-content {
  background-color: var(--color-paper);
  padding: var(--space-lg);
  border: 2px solid var(--color-ink);
  border-radius: 8px;
  margin-bottom: var(--space-xl);
}

.page-content h2 {
  margin-top: var(--space-lg);
}

/* Screen Reader Only */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border-width: 0;
}