body {
  font-family: 'Helvetica Neue', sans-serif;
  background-color: #f0f2f5;
  color: #333;
  margin: 0;
  padding: 1rem;
  box-sizing: border-box;
}

header {
  text-align: center;
  margin-bottom: 1.5rem;
}

h1 {
  font-size: 2rem;
  margin: 1rem 0;
  color: #333;
}

#lesson-selector {
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 0.5rem;
  margin-bottom: 2rem;
  max-width: 900px;
  margin-left: auto;
  margin-right: auto;
  padding: 0 1rem;
}

#lesson-selector button {
  background-color: #ffffff;
  color: #333;
  border: 1px solid #ccc;
  width: 60px;
  height: 40px;
  text-align: center;
  border-radius: 8px;
  font-size: 1rem;
  cursor: pointer;
  transition: 0.2s ease;
}

#lesson-selector button:hover {
  background-color: #f0f0f0;
  border-color: #999;
}

#lesson-selector button.active {
  background-color: #4CAF50;
  color: white;
  border-color: #4CAF50;
}

#audio-list {
  display: flex;
  flex-direction: column;
  gap: 1rem;
  max-width: 800px;
  margin: 0 auto;
  padding: 0 1rem;
}

.audio-card {
  background: #fff;
  border-radius: 12px;
  padding: 1rem;
  box-shadow: 0 2px 6px rgba(0, 0, 0, 0.08);
}

.audio-card .filename {
  font-weight: bold;
  margin-bottom: 0.5rem;
  font-size: 0.95rem;
}

.audio-card audio {
  width: 100%;
  outline: none;
}

footer {
  text-align: center;
  margin-top: 3rem;
  font-size: 0.85rem;
  color: #777;
}