:root {
  --primary-color: #2e7d32; /* 深绿色 */
  --secondary-color: #4caf50; /* 中等绿色 */
  --accent-color: #81c784; /* 浅绿色 */
  --text-color: #333;
  --background-color: #f5f5f5;
}

body {
  font-family: 'Georgia', serif;
  line-height: 1.6;
  color: var(--text-color);
  background-color: var(--background-color);
  margin: 0;
  padding: 0;
}

header {
  background-color: var(--primary-color);
  color: white;
  padding: 2rem;
  text-align: center;
}

h1 {
  font-size: 2.5rem;
  margin-bottom: 1rem;
}

nav ul {
  list-style: none;
  padding: 0;
  display: flex;
  justify-content: center;
  gap: 2rem;
}

nav a {
  color: white;
  text-decoration: none;
  font-weight: bold;
  transition: opacity 0.3s;
}

nav a:hover {
  opacity: 0.8;
}

section {
  padding: 2rem;
  max-width: 800px;
  margin: 0 auto;
}

h2 {
  color: var(--primary-color);
  border-bottom: 2px solid var(--accent-color);
  padding-bottom: 0.5rem;
  margin-bottom: 1.5rem;
}

ul {
  padding-left: 1.5rem;
}

a {
  color: var(--secondary-color);
  text-decoration: none;
}

a:hover {
  text-decoration: underline;
}

/* 时间轴样式 */
.timeline {
  position: relative;
  max-width: 800px;
  margin: 0 auto;
  padding: 40px 0;
}

.timeline::before {
  content: '';
  position: absolute;
  top: 0;
  bottom: 0;
  width: 4px;
  background: var(--accent-color);
  left: 50%;
  margin-left: -2px;
}

.timeline-item {
  padding: 20px 40px;
  position: relative;
  width: 50%;
}

.timeline-item:nth-child(odd) {
  left: 0;
  text-align: right;
}

.timeline-item:nth-child(even) {
  left: 50%;
}

.timeline-content {
  padding: 20px;
  background: white;
  border-radius: 6px;
  box-shadow: 0 2px 10px rgba(0,0,0,0.1);
  position: relative;
}

.timeline-content::before {
  content: '';
  position: absolute;
  width: 20px;
  height: 20px;
  background: var(--primary-color);
  border-radius: 50%;
  top: 20px;
}

.timeline-item:nth-child(odd) .timeline-content::before {
  right: -30px;
}

.timeline-item:nth-child(even) .timeline-content::before {
  left: -30px;
}

.timeline-content h3 {
  color: var(--primary-color);
  margin-top: 0;
}

.timeline-content h4 {
  color: var(--secondary-color);
  margin: 10px 0;
}

.timeline-content p {
  margin: 5px 0;
}

.research {
  color: var(--accent-color);
  font-style: italic;
}

/* 响应式设计 */
@media (max-width: 768px) {
  h1 {
    font-size: 2rem;
  }
  
  nav ul {
    flex-direction: column;
    gap: 1rem;
  }
  
  section {
    padding: 1rem;
  }
}