* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  font-family: 'Roboto', sans-serif;
  background: #f7f7f7;
  color: #333;
}
header {
  background: #2196f3;
  color: #fff;
  padding: 20px;
  text-align: center;
  box-shadow: 0 2px 5px rgba(0,0,0,0.1);
  position: relative;
}
.menu-toggle {
  display: none;
}
.container {
  display: flex;
  min-height: calc(100vh - 80px);
}
.sidebar {
  background: #1976d2;
  color: #fff;
  padding: 20px;
  width: 250px;
  flex-shrink: 0;
}
.sidebar h2 {
  margin-bottom: 20px;
  text-align: center;
  font-size: 1.3em;
}
.sidebar ul {
  list-style: none;
}
.lesson-item {
  display: flex;
  align-items: center;
  padding: 12px 15px;
  margin-bottom: 10px;
  border-radius: 6px;
  cursor: pointer;
  transition: background 0.3s, transform 0.2s;
}
.lesson-item:hover {
  transform: translateX(5px);
}
.lock-icon {
  margin-right: 10px;
  font-size: 1.2em;
}
.lesson-item.unlocked {
  background: #64b5f6;
  cursor: pointer;
}
.lesson-item.unlocked:hover {
  background: #42a5f5;
}
.lesson-item.locked {
  background: #90caf9;
  cursor: not-allowed;
  opacity: 0.8;
}
.content {
  flex-grow: 1;
  padding: 30px;
  background: #fff;
  overflow-y: auto;
}
#lessonContent {
  max-width: 800px;
  margin: 0 auto;
  padding-bottom: 40px;
}
#lessonContent h2 {
  color: #2196f3;
  margin-bottom: 15px;
  font-size: 1.8em;
}
.lesson-text p {
  margin-bottom: 15px;
  line-height: 1.6;
  font-size: 1.1em;
}
.exercise {
  margin-top: 25px;
  padding: 15px;
  border: 1px solid #ddd;
  border-radius: 6px;
  background: #fafafa;
}
.exercise pre {
  background: #f0f0f0;
  padding: 10px;
  border-radius: 4px;
  overflow-x: auto;
  font-family: monospace;
}
.exercise p.question {
  font-weight: bold;
  margin-bottom: 10px;
  font-size: 1.05em;
}
.exercise textarea {
  width: 100%;
  height: 80px;
  padding: 10px;
  margin-bottom: 10px;
  border: 1px solid #ccc;
  border-radius: 4px;
  resize: vertical;
  font-family: 'Roboto', sans-serif;
  font-size: 1em;
}
.exercise button {
  background: #2196f3;
  border: none;
  color: #fff;
  padding: 10px 20px;
  border-radius: 4px;
  cursor: pointer;
  transition: background 0.3s;
  font-size: 1em;
}
.exercise button:hover {
  background: #1976d2;
}
.exercise p.feedback {
  margin-top: 10px;
  font-weight: bold;
  font-size: 1em;
}
@media (max-width: 768px) {
  .container {
    flex-direction: column;
  }
  .sidebar {
    width: 250px;
    padding: 15px;
    position: fixed;
    top: 0;
    left: -250px;
    height: 100%;
    overflow-y: auto;
    transition: left 0.3s ease;
    z-index: 1050;
  }
  .sidebar.active {
    left: 0;
  }
  .content {
    padding: 20px;
  }
  .menu-toggle {
    display: block;
    background: none;
    border: none;
    color: #fff;
    font-size: 1.8em;
    cursor: pointer;
    position: absolute;
    top: 20px;
    left: 20px;
    z-index: 1100;
  }
}
@media (max-width: 480px) {
  header h1 {
    font-size: 1.5em;
  }
  .lesson-item {
    padding: 10px 12px;
    font-size: 0.95em;
  }
  .exercise textarea {
    height: 60px;
  }
}