/* UI风格变量 */
.ui-style-0 { --primary: #3498db; --secondary: #2c3e50; }
.ui-style-1 { --primary: #e74c3c; --secondary: #c0392b; }
.ui-style-2 { --primary: #2ecc71; --secondary: #27ae60; }
.ui-style-3 { --primary: #f39c12; --secondary: #e67e22; }
.ui-style-4 { --primary: #9b59b6; --secondary: #8e44ad; }
.ui-style-5 { --primary: #1abc9c; --secondary: #16a085; }
.ui-style-6 { --primary: #34495e; --secondary: #2c3e50; }
.ui-style-7 { --primary: #e67e22; --secondary: #d35400; }
.ui-style-8 { --primary: #3498db; --secondary: #2980b9; }
.ui-style-9 { --primary: #95a5a6; --secondary: #7f8c8d; }
.ui-style-10 { --primary: #c0392b; --secondary: #a93226; }
.ui-style-11 { --primary: #16a085; --secondary: #138d75; }
.ui-style-12 { --primary: #d35400; --secondary: #ba4a00; }
.ui-style-13 { --primary: #8e44ad; --secondary: #7d3c98; }
.ui-style-14 { --primary: #27ae60; --secondary: #229954; }
.ui-style-15 { --primary: #2980b9; --secondary: #21618c; }

/* 布局变体 */
.layout-A .container { max-width: 1200px; }
.layout-B .container { max-width: 1400px; }
.layout-C .container { max-width: 1000px; }
.layout-D .video-card { border-left: 4px solid var(--primary, #3498db); }
.layout-E .section-title { border-left: 6px solid var(--primary, #3498db); padding-left: 1rem; border-bottom: none; }
.layout-F .video-card { border-radius: 12px; }
.layout-G main { background: white; padding: 2rem; border-radius: 8px; margin-top: 2rem; }

/* 响应式增强 */
@media (max-width: 768px) {
  body { font-size: 14px; }
  .video-title { font-size: 1.2rem; }
  .section-title { font-size: 1.5rem; }
  .intro-section { padding: 1.5rem; }
}

/* 动画效果 */
.video-card {
  animation: fadeIn 0.5s ease-in;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(10px); }
  to { opacity: 1; transform: translateY(0); }
}

/* 返回顶部按钮 */
#backToTop {
  display: none;
  position: fixed;
  bottom: 30px;
  right: 30px;
  background: var(--primary, #3498db);
  color: white;
  width: 50px;
  height: 50px;
  border-radius: 50%;
  border: none;
  cursor: pointer;
  font-size: 1.5rem;
  box-shadow: 0 4px 8px rgba(0,0,0,0.2);
  transition: all 0.3s;
  z-index: 1000;
}

#backToTop:hover {
  transform: translateY(-5px);
  box-shadow: 0 6px 12px rgba(0,0,0,0.3);
}

@media (max-width: 768px) {
  #backToTop { width: 40px; height: 40px; font-size: 1.2rem; bottom: 20px; right: 20px; }
}