* { box-sizing: border-box; }
body {
  font-family: system-ui, sans-serif;
  margin: 0; padding: 0;
  background: linear-gradient(135deg,#eef2ff,#f9fafb);
  color: #1f2937;
}
header {
  text-align: center;
  padding: 1.5rem;
  background: #4f46e5;
  color: white;
}
main {
  display: flex;
  flex-direction: column;
  gap: 1rem;
  max-width: 1000px;
  margin: 1rem auto;
  padding: 0 1rem;
}
@media (min-width:768px){
  main { flex-direction: row; }
}
aside {
  background: white;
  border-radius: 1rem;
  padding: 1rem;
  flex: 1;
  box-shadow: 0 2px 6px rgba(0,0,0,0.1);
}
section {
  background: white;
  border-radius: 1rem;
  padding: 1rem;
  flex: 3;
  box-shadow: 0 2px 6px rgba(0,0,0,0.1);
}
button {
  cursor: pointer;
  border: 1px solid #d1d5db;
  border-radius: 0.5rem;
  background: #f9fafb;
  padding: 0.4rem 0.6rem;
}
button:hover { background: #eef2ff; }
.lesson-btn {
  width: 100%;
  text-align: left;
  margin-bottom: .5rem;
  padding: .5rem;
}
.lesson-btn.active { background: #e0e7ff; border-color:#818cf8; }
.progress {
  height: 6px;
  border-radius: 3px;
  background: #e5e7eb;
  margin-top: .5rem;
}
.progress-bar {
  height: 6px;
  border-radius: 3px;
  background: linear-gradient(90deg,#818cf8,#4f46e5);
}
.quiz button {
  display: block;
  margin: .3rem 0;
  width: 100%;
}
.feedback {
  margin-top: .3rem;
  font-size: .9rem;
}
footer {
  text-align: center;
  padding: 1rem;
  font-size: .8rem;
  color: #6b7280;
}