:root {
  --green: #16a34a;
  --dark-green: #15803d;
  --blue: #2563eb;
  --dark: #14213d;
  --text: #475569;
  --white: #ffffff;
  --light: #f8fafc;
  --border: #e2e8f0;
}

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html {
  scroll-behavior: smooth;
}

body {
  font-family: Arial, Helvetica, sans-serif;
  color: var(--dark);
  background: var(--light);
  line-height: 1.6;
}

button {
  font: inherit;
  cursor: pointer;
}

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

.container {
  width: min(1150px, 92%);
  margin: 0 auto;
}

:focus-visible {
  outline: 3px solid #f97316;
  outline-offset: 3px;
}

/* HEADER */

.site-header {
  background: var(--white);
  border-bottom: 1px solid var(--border);
}

.navbar {
  min-height: 78px;
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 20px;
}

.logo {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  font-size: 1.4rem;
  font-weight: 900;
  white-space: nowrap;
}

.logo > span:last-child {
  color: var(--green);
}

.logo-icon {
  background: var(--blue);
  color: var(--white);
  border-radius: 10px;
  padding: 8px;
  margin-right: 5px;
  font-size: 1rem;
}

.home-link {
  color: var(--blue);
  font-weight: 700;
}

.home-link:hover {
  text-decoration: underline;
}

/* WELCOME */

.welcome {
  padding: 60px 0 45px;
  text-align: center;
}

.eyebrow {
  color: var(--green);
  font-size: 0.85rem;
  font-weight: 800;
  letter-spacing: 1px;
}

.welcome h1 {
  font-size: clamp(2rem, 4vw, 3rem);
  margin: 12px 0;
}

.welcome p {
  color: var(--text);
  max-width: 650px;
  margin: 0 auto;
}

/* LEARNING LAYOUT */

.learning-layout {
  display: grid;
  grid-template-columns: 300px minmax(0, 1fr);
  gap: 25px;
  align-items: start;
}

.sidebar,
.lesson-panel {
  background: var(--white);
  border: 1px solid var(--border);
  border-radius: 20px;
  padding: 28px;
  box-shadow: 0 10px 30px rgba(15, 23, 42, 0.04);
}

.sidebar h2 {
  font-size: 1.25rem;
}

.sidebar > p {
  color: var(--text);
  font-size: 0.9rem;
  margin: 8px 0 22px;
}

.lesson-list {
  display: grid;
  gap: 10px;
}

.lesson-item {
  width: 100%;
  border: 1px solid var(--border);
  background: var(--light);
  padding: 13px;
  border-radius: 12px;
  text-align: left;
  font-weight: 700;
  transition: 0.2s;
}

.lesson-item:hover {
  border-color: var(--blue);
}

.lesson-item.active {
  background: #eff6ff;
  border-color: var(--blue);
  color: #1d4ed8;
}

.lesson-item.completed::after {
  content: " ✓";
  color: var(--green);
}

/* PROGRESS */

.progress-panel {
  margin-top: 30px;
  padding: 20px;
  border-radius: 15px;
  background: #f0fdf4;
}

.progress-panel h3 {
  font-size: 1rem;
  margin-bottom: 15px;
}

.progress-track {
  width: 100%;
  height: 12px;
  background: #dcfce7;
  border-radius: 30px;
  overflow: hidden;
}

.progress-fill {
  width: 0;
  height: 100%;
  background: var(--green);
  border-radius: 30px;
  transition: width 0.3s ease;
}

.progress-panel p {
  font-size: 0.85rem;
  margin-top: 12px;
  color: #166534;
  font-weight: 700;
}

/* NEXT LESSON AND ACHIEVEMENTS */

.next-lesson-panel,
.achievements-panel {
  margin-top: 22px;
  padding: 18px;
  border: 1px solid var(--border);
  border-radius: 15px;
  background: var(--white);
}

.next-lesson-panel h3,
.achievements-panel h3 {
  font-size: 1rem;
  margin-bottom: 10px;
}

.next-lesson-panel > p,
.achievements-intro {
  color: var(--text);
  font-size: 0.9rem;
  margin-bottom: 14px;
  overflow-wrap: anywhere;
}

.next-lesson-panel .btn {
  width: 100%;
  padding: 10px 12px;
  font-size: 0.9rem;
}

.achievement-list {
  display: grid;
  gap: 10px;
}

.achievement-card {
  display: flex;
  align-items: flex-start;
  gap: 10px;
  padding: 12px;
  border: 1px solid var(--border);
  border-radius: 12px;
}

.achievement-card.unlocked {
  background: #f0fdf4;
  border-color: #86efac;
}

.achievement-card.locked {
  background: #f8fafc;
  color: #64748b;
}

.achievement-icon {
  font-size: 1.4rem;
  line-height: 1.4;
}

.achievement-details {
  min-width: 0;
}

.achievement-details h4 {
  font-size: 0.9rem;
  margin-bottom: 3px;
}

.achievement-details p {
  font-size: 0.8rem;
  line-height: 1.45;
  overflow-wrap: anywhere;
}

/* LESSON CONTENT */

.lesson-top {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 10px;
  margin-bottom: 20px;
}

.lesson-number {
  color: var(--blue);
  font-weight: 800;
}

.level-badge {
  background: #dcfce7;
  color: #166534;
  border-radius: 30px;
  padding: 6px 12px;
  font-size: 0.75rem;
  font-weight: 800;
}

.lesson-panel h2 {
  font-size: clamp(1.6rem, 3vw, 2.2rem);
  margin-bottom: 15px;
}

.lesson-description {
  color: var(--text);
  margin-bottom: 25px;
}

.example-box {
  background: #0f172a;
  color: #e2e8f0;
  border-radius: 15px;
  overflow: hidden;
  margin-bottom: 25px;
}

.example-heading {
  background: #1e293b;
  padding: 13px 20px;
  font-size: 0.85rem;
  font-weight: 700;
}

.example-box pre {
  padding: 23px;
  overflow-x: auto;
  font-size: 0.95rem;
  line-height: 1.8;
}

.explanation-box {
  background: #f0fdf4;
  border-left: 4px solid var(--green);
  border-radius: 10px;
  padding: 20px;
  margin-bottom: 30px;
}

.explanation-box h3 {
  margin-bottom: 10px;
}

.explanation-box p {
  color: var(--text);
}

/* QUIZ */

.quiz-section {
  border-top: 1px solid var(--border);
  padding-top: 25px;
}

.quiz-section h3 {
  font-size: 1.3rem;
  margin-bottom: 10px;
}

.quiz-section > p {
  margin-bottom: 17px;
}

.quiz-options {
  display: grid;
  gap: 10px;
}

.quiz-option {
  padding: 13px 15px;
  border: 2px solid var(--border);
  background: var(--white);
  border-radius: 10px;
  text-align: left;
  font-weight: 700;
}

.quiz-option:hover:not(:disabled) {
  border-color: var(--blue);
  background: #eff6ff;
}

.quiz-option.correct {
  background: #dcfce7;
  border-color: var(--green);
}

.quiz-option.incorrect {
  background: #fee2e2;
  border-color: #dc2626;
}

.quiz-option:disabled {
  cursor: default;
}

.quiz-feedback {
  min-height: 25px;
  margin-top: 15px;
  font-weight: 700;
}

.quiz-feedback.success {
  color: #166534;
}

.quiz-feedback.error {
  color: #b91c1c;
}

/* NAVIGATION BUTTONS */

.lesson-actions {
  display: flex;
  justify-content: space-between;
  flex-wrap: wrap;
  gap: 15px;
  margin-top: 30px;
}

.btn {
  border: 0;
  border-radius: 10px;
  padding: 12px 20px;
  font-weight: 800;
}

.btn-primary {
  background: var(--green);
  color: var(--white);
}

.btn-primary:hover:not(:disabled) {
  background: var(--dark-green);
}

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

.btn:disabled {
  opacity: 0.45;
  cursor: not-allowed;
}

/* ENCOURAGEMENT */

.encouragement {
  text-align: center;
  padding: 65px 20px;
}

.encouragement h2 {
  font-size: clamp(1.4rem, 3vw, 2rem);
  margin-bottom: 10px;
}

.encouragement p {
  color: var(--text);
}

/* FOOTER */

.site-footer {
  background: #0f172a;
  color: #cbd5e1;
  padding: 30px 20px;
  text-align: center;
}

.site-footer p {
  margin: 5px 0;
  font-size: 0.9rem;
}

/* RESPONSIVE */

@media (max-width: 800px) {
  .learning-layout {
    grid-template-columns: 1fr;
  }

  .lesson-list {
    grid-template-columns: repeat(2, 1fr);
  }
}

@media (max-width: 520px) {
  .navbar {
    flex-wrap: wrap;
    justify-content: center;
    padding: 15px 0;
  }

  .logo {
    font-size: 1.1rem;
  }

  .welcome {
    padding: 45px 0 30px;
  }

  .sidebar,
  .lesson-panel {
    padding: 20px;
  }

  .lesson-list {
    grid-template-columns: 1fr;
  }

  .lesson-actions {
    flex-direction: column;
  }

  .lesson-actions .btn {
    width: 100%;
  }
}

@media (prefers-reduced-motion: reduce) {
  *,
  *::before,
  *::after {
    transition-duration: 0.01ms !important;
  }
}