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

body {
  background: #f8f9fa;
  padding: 40px 0;
}

.container {
  max-width: 1000px;
  margin: 0 auto;
  background: #ffffff;
  padding: 45px;
  border-radius: 16px;
  box-shadow: 0 4px 20px rgba(0, 0, 0, 0.05);
}

h1 {
  text-align: center;
  font-size: 26px;
  color: #222;
  margin-bottom: 8px;
}

.subtitle {
  text-align: center;
  color: #666;
  font-size: 15px;
  margin-bottom: 32px;
}

/* 🎯 美观版按钮组 */
.controls {
  display: flex;
  justify-content: center;
  gap: 14px;
  margin-bottom: 40px;
}

.controls button {
  padding: 12px 24px;
  border: none;
  border-radius: 10px;
  font-size: 15px;
  font-weight: 500;
  cursor: pointer;
  transition: all 0.25s ease;
  background: #ffffff;
  color: #333;
  border: 1px solid #e0e6ed;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.04);
}

.controls button:hover {
  transform: translateY(-2px);
  box-shadow: 0 4px 12px rgba(64, 158, 255, 0.15);
  border-color: #409eff;
  color: #409eff;
}

.controls button.active {
  background: #409eff;
  color: #fff;
  border-color: #409eff;
}

#visualization {
  display: flex;
  justify-content: center;
  margin-bottom: 45px;
}

/* 研究问题框 */
.question-box {
  background: #eff8ff;
  border-left: 5px solid #409eff;
  padding: 18px 20px;
  border-radius: 10px;
  margin-bottom: 30px;
}

.question-box h3 {
  margin-bottom: 6px;
  color: #222;
}

/* 作业文档 */
.doc-box {
  line-height: 1.8;
  color: #333;
  padding-top: 20px;
  border-top: 1px solid #eee;
}

.doc-box h2 {
  font-size: 20px;
  margin-bottom: 16px;
  color: #222;
}

.doc-box p {
  margin-bottom: 12px;
}

/*  tooltip */
.tooltip {
  position: absolute;
  background: rgba(0, 0, 0, 0.75);
  color: #fff;
  padding: 8px 12px;
  border-radius: 8px;
  font-size: 13px;
  pointer-events: none;
  opacity: 0;
  transition: opacity 0.2s;
}