:root {
  --bg-dark: #0f172a;
  --bg-darker: #020617;
  --primary: #3b82f6;
  --secondary: #06b6d4;
  --accent: #8b5cf6;
  --text-main: #f8fafc;
  --text-muted: #94a3b8;
  --glass-bg: rgba(255, 255, 255, 0.05);
  --glass-border: rgba(255, 255, 255, 0.1);
  --card-shadow: 0 8px 32px 0 rgba(0, 0, 0, 0.37);
}

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
}

body {
  background: var(--bg-darker);
  color: var(--text-main);
  line-height: 1.6;
  min-height: 100vh;
  overflow-x: hidden;
}

/* 动态科技背景 */
body::before {
  content: "";
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: 
    radial-gradient(circle at 20% 30%, rgba(59, 130, 246, 0.1) 0%, transparent 40%),
    radial-gradient(circle at 80% 70%, rgba(139, 92, 246, 0.1) 0%, transparent 40%);
  z-index: -1;
}

.header {
  background: rgba(15, 23, 42, 0.8);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--glass-border);
  color: white;
  padding: 1.25rem 2rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
  position: sticky;
  top: 0;
  z-index: 100;
}

.logo {
  font-size: 1.5rem;
  font-weight: 800;
  background: linear-gradient(135deg, var(--primary), var(--secondary));
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  letter-spacing: -0.5px;
}

.nav a {
  color: var(--text-muted);
  margin-left: 2rem;
  text-decoration: none;
  font-weight: 500;
  transition: all 0.3s ease;
  font-size: 0.95rem;
}

.nav a:hover {
  color: var(--primary);
  text-shadow: 0 0 8px rgba(59, 130, 246, 0.4);
}

.banner {
  padding: 8rem 2rem;
  text-align: center;
  position: relative;
  overflow: hidden;
}

.banner h1 {
  font-size: 3.5rem;
  font-weight: 800;
  margin-bottom: 1.5rem;
  background: linear-gradient(to right, #fff, #94a3b8);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  letter-spacing: -1px;
}

.banner p {
  font-size: 1.25rem;
  color: var(--text-muted);
  margin-bottom: 2.5rem;
  letter-spacing: 2px;
  text-transform: uppercase;
}

.btn {
  background: linear-gradient(135deg, var(--primary), var(--secondary));
  color: white;
  padding: 0.8rem 2rem;
  border: none;
  border-radius: 50px;
  cursor: pointer;
  text-decoration: none;
  display: inline-block;
  font-weight: 600;
  transition: all 0.3s ease;
  box-shadow: 0 4px 15px rgba(59, 130, 246, 0.3);
}

.btn:hover {
  transform: translateY(-2px);
  box-shadow: 0 8px 25px rgba(59, 130, 246, 0.5);
}

.section {
  max-width: 1100px;
  margin: 4rem auto;
  padding: 0 2rem;
}

.section h2 {
  font-size: 2rem;
  margin-bottom: 2rem;
  text-align: center;
  position: relative;
}

.section h2::after {
  content: "";
  display: block;
  width: 50px;
  height: 4px;
  background: var(--primary);
  margin: 10px auto;
  border-radius: 2px;
}

.chapter {
  margin-bottom: 1.5rem;
}

.chapter-header {
  background: var(--glass-bg);
  padding: 1.25rem 2rem;
  border-radius: 16px;
  cursor: pointer;
  display: flex;
  justify-content: space-between;
  align-items: center;
  border: 1px solid var(--glass-border);
  transition: all 0.3s ease;
  font-size: 1.2rem;
  color: var(--secondary);
}

.chapter-header:hover {
  background: rgba(255, 255, 255, 0.08);
  border-color: var(--primary);
}

.chapter.active .chapter-header {
  border-bottom-left-radius: 0;
  border-bottom-right-radius: 0;
  border-color: var(--primary);
  background: rgba(59, 130, 246, 0.1);
}

.chapter-header .arrow {
  font-size: 0.8rem;
  transition: transform 0.3s ease;
  color: var(--text-muted);
}

.chapter.active .arrow {
  transform: rotate(180deg);
}

.chapter-content {
  display: none;
  background: rgba(0, 0, 0, 0.2);
  padding: 2rem;
  border: 1px solid var(--glass-border);
  border-top: none;
  border-bottom-left-radius: 16px;
  border-bottom-right-radius: 16px;
}

.chapter.active .chapter-content {
  display: block;
  animation: slideDown 0.3s ease-out;
}

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

/* 多级折叠样式：节次级（二级） */
.section-item {
  margin: 0.75rem 0;
}

.section-header {
  background: rgba(255, 255, 255, 0.03);
  padding: 1rem 1.5rem;
  border-radius: 12px;
  cursor: pointer;
  display: flex;
  justify-content: space-between;
  align-items: center;
  border: 1px solid var(--glass-border);
  transition: all 0.3s ease;
  font-size: 1rem;
  color: var(--text-main);
}

.section-header:hover {
  background: rgba(255, 255, 255, 0.08);
  border-color: var(--secondary);
}

.section-item.active .section-header {
  border-bottom-left-radius: 0;
  border-bottom-right-radius: 0;
  background: rgba(6, 182, 212, 0.1);
  border-color: var(--secondary);
}

.section-item.active .arrow {
  transform: rotate(180deg);
}

.section-content {
  display: none;
  background: rgba(0, 0, 0, 0.3);
  padding: 1.5rem;
  border: 1px solid var(--glass-border);
  border-top: none;
  border-bottom-left-radius: 12px;
  border-bottom-right-radius: 12px;
}

.section-item.active .section-content {
  display: block;
  animation: slideDown 0.3s ease-out;
}

/* 用户个人信息弹窗 */
.profile-modal {
  display: none;
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background: rgba(0, 0, 0, 0.6);
  backdrop-filter: blur(8px);
  z-index: 1000;
  align-items: center;
  justify-content: center;
}

.profile-card {
  background: var(--bg-dark);
  border: 1px solid var(--glass-border);
  border-radius: 24px;
  padding: 3rem;
  width: 100%;
  max-width: 450px;
  box-shadow: var(--card-shadow);
  text-align: center;
  animation: modalFadeIn 0.3s ease;
}

@keyframes modalFadeIn {
  from { opacity: 0; transform: scale(0.9); }
  to { opacity: 1; transform: scale(1); }
}

.profile-avatar {
  width: 80px;
  height: 80px;
  background: linear-gradient(135deg, var(--primary), var(--accent));
  border-radius: 50%;
  margin: 0 auto 1.5rem;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 2rem;
  color: white;
  box-shadow: 0 0 20px rgba(59, 130, 246, 0.4);
}

.profile-info-row {
  display: flex;
  justify-content: space-between;
  padding: 1rem 0;
  border-bottom: 1px solid var(--glass-border);
}

.profile-info-label {
  color: var(--text-muted);
  font-size: 0.9rem;
}

.profile-info-value {
  color: var(--text-main);
  font-weight: 600;
}

.experiment-grid {
  background: var(--glass-bg);
  backdrop-filter: blur(8px);
  -webkit-backdrop-filter: blur(8px);
  padding: 1.5rem;
  border-radius: 16px;
  margin: 1rem 0;
  cursor: pointer;
  border: 1px solid var(--glass-border);
  transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
  box-shadow: var(--card-shadow);
}

.card:hover {
  background: rgba(255, 255, 255, 0.08);
  transform: scale(1.02);
  border-color: var(--primary);
  box-shadow: 0 0 20px rgba(59, 130, 246, 0.2);
}

.card.lock {
  opacity: 0.5;
  cursor: not-allowed;
  filter: grayscale(1);
}

.form-box {
  max-width: 450px;
  margin: 6rem auto;
  background: var(--glass-bg);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  padding: 3rem;
  border-radius: 24px;
  border: 1px solid var(--glass-border);
  box-shadow: var(--card-shadow);
}

.form-box h2 {
  margin-bottom: 2rem;
  font-size: 1.75rem;
  font-weight: 700;
  text-align: center;
}

.form-box input, .form-box select {
  width: 100%;
  padding: 0.8rem 1.2rem;
  margin: 1rem 0;
  background: rgba(0, 0, 0, 0.2);
  border: 1px solid var(--glass-border);
  border-radius: 12px;
  color: white;
  transition: all 0.3s ease;
}

.form-box input:focus {
  outline: none;
  border-color: var(--primary);
  background: rgba(0, 0, 0, 0.3);
}

.vip-cards {
  display: flex;
  gap: 2rem;
  flex-wrap: wrap;
  justify-content: center;
  margin-top: 3rem;
}

.vip-card {
  background: var(--glass-bg);
  backdrop-filter: blur(8px);
  -webkit-backdrop-filter: blur(8px);
  padding: 3rem 2rem;
  border-radius: 24px;
  width: 300px;
  text-align: center;
  border: 1px solid var(--glass-border);
  transition: all 0.3s ease;
  box-shadow: var(--card-shadow);
  position: relative;
}

.vip-card:hover {
  transform: translateY(-10px);
  border-color: var(--accent);
}

.price {
  font-size: 2.5rem;
  color: var(--text-main);
  font-weight: 800;
  margin: 1.5rem 0;
}

.price span {
  font-size: 1rem;
  color: var(--text-muted);
  font-weight: 400;
}

.pay-info {
  max-width: 600px;
  margin: 3rem auto;
  background: rgba(251, 191, 36, 0.1);
  border: 1px solid rgba(251, 191, 36, 0.3);
  color: #fbbf24;
  padding: 1.5rem;
  border-radius: 16px;
  text-align: center;
  display: none;
}

.grade-tabs {
  display: flex;
  justify-content: center;
  gap: 1rem;
  margin-bottom: 3rem;
  background: var(--glass-bg);
  padding: 0.5rem;
  border-radius: 50px;
  border: 1px solid var(--glass-border);
  max-width: fit-content;
  margin-left: auto;
  margin-right: auto;
}

.tab-btn {
  background: transparent;
  color: var(--text-muted);
  border: none;
  padding: 0.75rem 2rem;
  border-radius: 50px;
  cursor: pointer;
  font-weight: 600;
  transition: all 0.3s ease;
}

.tab-btn.active {
  background: var(--primary);
  color: white;
  box-shadow: 0 4px 15px rgba(59, 130, 246, 0.3);
}

.grade-content {
  display: none;
  animation: fadeIn 0.5s ease;
}

.grade-content.active {
  display: block;
}

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

.experiment-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: 1.5rem;
}

.card-icon {
  font-size: 2.5rem;
  margin-bottom: 1rem;
}

.tag {
  display: inline-block;
  padding: 0.25rem 0.75rem;
  border-radius: 20px;
  font-size: 0.75rem;
  background: rgba(59, 130, 246, 0.2);
  color: var(--primary);
  margin-top: 0.5rem;
}

.tag.vip {
  background: rgba(139, 92, 246, 0.2);
  color: var(--accent);
}

.glow-sphere {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 400px;
  height: 400px;
  background: radial-gradient(circle, rgba(59, 130, 246, 0.15) 0%, transparent 70%);
  z-index: -1;
  pointer-events: none;
}

/* 自定义滚动条 */
::-webkit-scrollbar {
  width: 8px;
}

::-webkit-scrollbar-track {
  background: var(--bg-darker);
}

::-webkit-scrollbar-thumb {
  background: var(--glass-border);
  border-radius: 4px;
}

::-webkit-scrollbar-thumb:hover {
  background: var(--primary);
}

@media (max-width: 768px) {
  .banner h1 {
    font-size: 2.2rem;
  }
  .banner p {
    font-size: 1rem;
  }
  .header {
    flex-direction: column;
    gap: 1rem;
    padding: 1rem;
  }
  .nav a {
    margin: 0 0.75rem;
    font-size: 0.85rem;
  }
  .form-box {
    padding: 2rem 1.5rem;
  }
}