:root {
  --header-bg: #2b7a8d;
  --bg-color: #f3f6f9;
  --card-bg: #ffffff;
  --text-main: #2c3e50;
  --primary-blue: #2980b9;
  --active-blue: #236794;
  --yellow-bg: #fdf6e2;
  --yellow-border: #f39c12;
  --gray-border: #cbd5e1;
}

* { 
  box-sizing: border-box; 
  margin: 0; 
  padding: 0; 
}

body { 
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif; 
  background-color: var(--bg-color); 
  color: var(--text-main); 
  line-height: 1.5;
}

/* HEADER FIX */
.header { 
  background-color: var(--header-bg); 
  color: #fff; 
  padding: 16px 40px; 
  display: flex; 
  justify-content: space-between; 
  align-items: center; 
}
.header h1 { font-size: 1.2rem; font-weight: 700; text-transform: uppercase; }
.header p { font-size: 0.8rem; opacity: 0.8; }
.header-right { font-weight: 600; font-size: 0.95rem; }

/* LAYOUT CONTAINER FIX */
.main-layout { 
  display: flex; 
  gap: 24px; 
  padding: 30px 40px; 
  max-width: 1400px; 
  margin: 0 auto; 
  align-items: flex-start;
}

/* SIDEBAR FIX */
.sidebar { 
  width: 280px; 
  flex-shrink: 0; 
  background: var(--card-bg); 
  border-radius: 8px; 
  padding: 20px 0; 
  box-shadow: 0 1px 3px rgba(0,0,0,0.05); 
}
.chapter-title { 
  font-size: 0.8rem; 
  font-weight: 700; 
  color: var(--primary-blue); 
  text-transform: uppercase; 
  padding: 12px 20px 6px; 
}
.lesson-link { 
  display: block; 
  padding: 10px 20px; 
  color: #555; 
  text-decoration: none; 
  font-size: 0.9rem; 
  transition: all 0.2s; 
}
.lesson-link:hover { background-color: #f8fafc; color: var(--primary-blue); }
.lesson-link.active { 
  background-color: #ebf5fb; 
  color: var(--primary-blue); 
  font-weight: 600; 
  border-radius: 6px; 
  margin: 0 8px; 
}

/* CONTENT AREA FIX */
.content-area { 
  flex: 1; 
  min-width: 0; /* Chống vỡ flex child khi chứa nội dung rộng */
}

.lesson-header { 
  background: var(--card-bg); 
  padding: 24px; 
  border-radius: 8px; 
  margin-bottom: 20px; 
  box-shadow: 0 1px 3px rgba(0,0,0,0.05); 
}
.lesson-header h2 { font-size: 1.5rem; color: #1e3a8a; margin-bottom: 8px; }
.lesson-meta { color: #64748b; font-size: 0.9rem; }

/* TAB BAR FIX */
.tab-bar { display: flex; gap: 12px; margin-bottom: 20px; }
.tab-btn { 
  flex: 1; 
  padding: 12px; 
  background: #e2e8f0; 
  border: none; 
  border-radius: 6px; 
  font-weight: 600; 
  font-size: 0.85rem; 
  color: #475569; 
  cursor: pointer; 
  transition: all 0.2s; 
  text-align: center; 
}
.tab-btn.active { background: var(--primary-blue); color: white; }

/* CARD & CALLOUT FIX */
.content-card { 
  background: var(--card-bg); 
  padding: 24px; 
  border-radius: 8px; 
  box-shadow: 0 1px 3px rgba(0,0,0,0.05); 
}
.section-title { font-size: 1.15rem; font-weight: 700; margin-bottom: 16px; }

.callout-yellow { 
  background-color: var(--yellow-bg); 
  border-left: 4px solid var(--yellow-border); 
  padding: 16px; 
  border-radius: 4px; 
  margin: 16px 0; 
}

/* FORM ELEMENTS FIX (ÉP KHUNG RỘNG 100%) */
.content-card textarea {
  display: block !important;
  width: 100% !important;
  min-height: 110px !important;
  margin-top: 14px !important;
  padding: 12px !important;
  border: 1px solid var(--gray-border) !important;
  border-radius: 8px !important;
  font-family: inherit !important;
  font-size: 0.95rem !important;
  box-sizing: border-box !important;
  resize: vertical !important;
}

.content-card button {
  display: inline-block !important;
  background-color: var(--primary-blue) !important;
  color: #ffffff !important;
  border: none !important;
  padding: 10px 20px !important;
  border-radius: 6px !important;
  font-weight: 600 !important;
  font-size: 0.9rem !important;
  cursor: pointer !important;
  margin-top: 14px !important;
}

.content-card button:hover {
  background-color: var(--active-blue) !important;
}

.preview-box {
  margin-top: 16px;
  padding: 14px;
  background-color: #f8fafc;
  border-left: 4px solid var(--primary-blue);
  border-radius: 6px;
  display: none;
}

/* TRẮC NGHIỆM UI */
.quiz-q {
  background: #f8fafc;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
  padding: 16px;
  margin-bottom: 16px;
}
.quiz-q .q-title {
  font-weight: 600;
  margin-bottom: 12px;
}
.quiz-options {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
  gap: 10px;
}
.quiz-options label {
  display: flex;
  align-items: center;
  gap: 8px;
  background: #ffffff;
  padding: 8px 12px;
  border: 1px solid #cbd5e1;
  border-radius: 6px;
  cursor: pointer;
  transition: all 0.2s;
}
.quiz-options label:hover {
  border-color: var(--primary-blue);
  background: #f0f9ff;
}
.q-feedback {
  margin-top: 8px;
  font-size: 0.9rem;
}
.quiz-result-box {
  background: #e0f2fe;
  border: 2px solid #0284c7;
  padding: 16px;
  border-radius: 8px;
  font-size: 1.1rem;
  text-align: center;
  margin-bottom: 20px;
  display: none;
}