:root {
  --primary-color: #4caf50;
  --primary-color-dark: #3a8a3f;
  --bg-color: #f5f5f5;
  --card-bg: #ffffff;
  --text-color: #333333;
  --muted-text: #777777;
  --border-radius-lg: 16px;
  --border-radius-md: 12px;
  --shadow-soft: 0 8px 20px rgba(0, 0, 0, 0.08);
}

* {
  box-sizing: border-box;
}

html,
body {
  margin: 0;
  padding: 0;
  height: 100%;
}

body {
  font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  background-color: var(--bg-color);
  color: var(--text-color);
}

.app-header {
  padding: 16px 20px 8px;
  background: linear-gradient(135deg, #66bb6a, #4caf50);
  color: #ffffff;
  text-align: left;
}

.app-title {
  margin: 0;
  font-size: 1.4rem;
  font-weight: 700;
}

.app-subtitle {
  margin: 4px 0 0;
  font-size: 0.85rem;
  opacity: 0.9;
}

.app-main {
  max-width: 640px;
  margin: 0 auto;
  padding: 12px 16px 24px;
}

.view {
  display: none;
}

.view-home {
  display: block;
}

.home-tip {
  margin: 4px 0 12px;
  font-size: 0.95rem;
  color: var(--muted-text);
}

.subjects-grid {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 12px;
}

.subject-card {
  border: none;
  border-radius: var(--border-radius-lg);
  padding: 14px 10px;
  background: linear-gradient(135deg, #ffffff, #f1f8e9);
  box-shadow: var(--shadow-soft);
  text-align: left;
  display: flex;
  flex-direction: column;
  gap: 2px;
  cursor: pointer;
  transition: transform 0.1s ease, box-shadow 0.1s ease;
}

.subject-card:active {
  transform: translateY(1px);
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.06);
}

.subject-icon {
  font-size: 1.6rem;
}

.subject-name {
  font-size: 1rem;
  font-weight: 600;
}

.subject-desc {
  font-size: 0.8rem;
  color: var(--muted-text);
}

.view-card {
  display: block;
}

.back-btn {
  border: none;
  background: transparent;
  color: var(--primary-color);
  font-size: 0.9rem;
  padding: 4px 0;
  cursor: pointer;
}

.card-wrapper {
  margin-top: 8px;
  border-radius: var(--border-radius-lg);
  background-color: var(--card-bg);
  box-shadow: var(--shadow-soft);
  padding: 16px 16px 18px;
}

.card-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
}

.card-header-main {
  display: flex;
  align-items: center;
  gap: 10px;
}

.card-icon {
  font-size: 1.8rem;
}

.card-header-text {
  display: flex;
  flex-direction: column;
}

.card-title {
  font-size: 1.1rem;
  font-weight: 600;
}

.card-subtitle {
  font-size: 0.8rem;
  color: var(--muted-text);
}

.voice-btn {
  border: none;
  border-radius: 999px;
  padding: 6px 14px;
  font-size: 0.85rem;
  background: linear-gradient(135deg, #7e57c2, #5c6bc0);
  color: #ffffff;
  cursor: pointer;
  box-shadow: 0 4px 12px rgba(92, 107, 192, 0.4);
}

.card-content {
  margin-top: 14px;
  font-size: 1rem;
}

.card-content .big-char {
  font-size: 3rem;
  font-weight: 700;
  text-align: center;
  margin-bottom: 6px;
}

.card-content .pinyin {
  font-size: 1.2rem;
  text-align: center;
  color: #666666;
  margin-bottom: 6px;
}

.card-content .meaning {
  font-size: 1rem;
  text-align: center;
}

.card-content .math-question {
  font-size: 1.6rem;
  text-align: center;
  margin-bottom: 8px;
}

.card-content .english-emoji {
  font-size: 2.2rem;
  text-align: center;
}

.card-content .english-word {
  font-size: 1.8rem;
  text-align: center;
  margin-top: 4px;
}

.card-content .pronunciation {
  font-size: 0.95rem;
  text-align: center;
  color: #666666;
}

.card-content .english-chinese {
  font-size: 1rem;
  text-align: center;
  margin-top: 4px;
}

.card-content .science-title,
.card-content .story-title {
  font-size: 1.05rem;
  font-weight: 600;
  margin-bottom: 6px;
}

.card-content .science-content,
.card-content .story-content {
  font-size: 0.95rem;
  line-height: 1.5;
}

.card-content .story-moral {
  margin-top: 8px;
  font-size: 0.9rem;
  color: var(--muted-text);
}

.math-controls {
  margin-top: 10px;
  text-align: center;
}

.math-answer {
  margin-top: 6px;
  font-size: 1rem;
}

.card-actions {
  margin-top: 14px;
  display: flex;
  justify-content: center;
}

.primary-btn,
.secondary-btn {
  border-radius: 999px;
  padding: 10px 24px;
  font-size: 0.95rem;
  border: none;
  cursor: pointer;
}

.primary-btn {
  background: linear-gradient(135deg, var(--primary-color), var(--primary-color-dark));
  color: #ffffff;
  box-shadow: var(--shadow-soft);
}

.secondary-btn {
  background-color: #eeeeee;
  color: #555555;
}

.toast {
  position: fixed;
  left: 50%;
  bottom: 20px;
  transform: translateX(-50%);
  background: rgba(0, 0, 0, 0.8);
  color: #ffffff;
  padding: 8px 16px;
  border-radius: 999px;
  font-size: 0.85rem;
  opacity: 0;
  pointer-events: none;
  transition: opacity 0.2s ease;
}

.toast.show {
  opacity: 1;
}

.tts-audio {
  width: 100%;
  margin-top: 8px;
}

@media (min-width: 768px) {
  .app-main {
    padding-top: 20px;
  }

  .card-wrapper {
    padding: 18px 20px 20px;
  }
}