* {
  box-sizing: border-box;
}

body {
  margin: 0;
  background: #f7f8fc;
  color: #222;
}

.lesson-page {
  display: flex;
  min-height: 100vh;
}

.lesson-sidebar {
  width: 260px;
  background: #ffffff;
  border-right: 1px solid #e5e7eb;
  padding: 24px;
}

.lesson-sidebar h2 {
  margin-top: 0;
  margin-bottom: 24px;
}

.unit-block {
  margin-bottom: 24px;
}

.unit-block h3 {
  margin-bottom: 10px;
  font-size: 1rem;
  color: #444;
}

.unit-block ul {
  list-style: none;
  padding: 0;
  margin: 0;
}

.unit-block li {
  margin-bottom: 8px;
}

.unit-block a {
  text-decoration: none;
  color: #333;
}

.unit-block a:hover {
  color: #2563eb;
}

.selected-lesson {
  color: #2563eb !important;
  font-weight: 700;
}

.lesson-main {
  flex: 1;
  padding: 40px;
}

.lesson-template-main {
  max-width: 900px;
  margin: 0 auto;
}

.lesson-header {
  margin-bottom: 24px;
}

.lesson-header h1 {
  margin: 0 0 10px;
  font-size: 2rem;
}

.lesson-header p {
  margin: 0 0 10px;
  line-height: 1.6;
  color: #555;
}

.start-container {
  margin-top: 32px;
  text-align: right;
}

#start-btn {
  padding: 12px 24px;
  background-color: #2d6cdf;
  color: white;
  border: none;
  border-radius: 20px;
  font-size: 1rem;
  cursor: pointer;
}

#start-btn:hover {
  background-color: #1f4fbf;
}

.mode-switch {
  display: flex;
  gap: 12px;
  margin-bottom: 24px;
  flex-wrap: wrap;
}

.mode-btn {
  padding: 10px 18px;
  border: none;
  border-radius: 999px;
  background: #e5e7eb;
  cursor: pointer;
}

.mode-btn.active {
  background: #2563eb;
  color: white;
}

.mode-section {
  display: none;
}

.active-section {
  display: block;
}

.content-card {
  background: white;
  border-radius: 16px;
  padding: 22px;
  margin-bottom: 18px;
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.06);
}

.content-card h2 {
  margin-top: 0;
  margin-bottom: 10px;
}

.content-card p,
.content-card li {
  line-height: 1.7;
  color: #444;
}

#course-title {
  cursor: pointer;
}

#course-title:hover {
  color: #2563eb;
}

@media (max-width: 900px) {
  .lesson-page {
    flex-direction: column;
  }

  .lesson-sidebar {
    width: 100%;
    border-right: none;
    border-bottom: 1px solid #e5e7eb;
  }

  .lesson-main {
    padding: 24px;
  }

  .start-container {
    text-align: left;
  }
}