body {
  margin: 0;
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
  color: #111;
  background: #fff;
}
#app {
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}
.home-link {
  position: absolute;
  top: 12px;
  left: 16px;
  text-decoration: none;
  color: #2563eb;
  font-size: 14px;
  z-index: 10;
}
.lesson-header {
  padding: 24px 16px;
  border-bottom: 1px solid #e5e7eb;
}
.lesson-header h1 {
  margin: 0 0 8px;
  font-size: 26px;
}
.lesson-intro {
  margin: 0;
  color: #4b5563;
  line-height: 1.5;
}
.step-list {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  padding: 12px 16px;
  border-bottom: 1px solid #e5e7eb;
}
.step-item {
  background: #eef2ff;
  border: 1px solid #c7d2fe;
  border-radius: 6px;
  padding: 6px 12px;
  font-size: 13px;
  cursor: pointer;
  transition: background 0.2s;
}
.step-item:hover {
  background: #e0e7ff;
}
.step-item.active {
  background: #2563eb;
  color: #fff;
  border-color: #1d4ed8;
}
.canvas-area {
  flex: 1;
  display: flex;
  flex-direction: column;
  padding: 16px;
  gap: 12px;
  overflow: auto;
}
.scene {
  background: #f8fafc;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
  padding: 12px;
}
.network-svg {
  display: block;
  width: 100%;
  height: auto;
  max-height: 420px;
}
.controls-area {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  align-items: center;
}
.control-btn {
  background: #2563eb;
  color: #fff;
  border: none;
  border-radius: 6px;
  padding: 8px 14px;
  font-size: 14px;
  cursor: pointer;
}
.control-btn:hover {
  background: #1d4ed8;
}
.settings-area {
  display: flex;
  gap: 14px;
  font-size: 13px;
}
.packet-detail {
  background: #f8fafc;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
  padding: 14px;
}
.packet-detail h3 {
  margin: 0 0 8px;
  font-size: 15px;
}
.mono {
  margin: 0;
  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
  font-size: 12px;
  line-height: 1.5;
  white-space: pre-wrap;
}
.lesson-footer {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 16px;
  border-top: 1px solid #e5e7eb;
}
.nav-btn {
  background: #fff;
  border: 1px solid #d1d5db;
  border-radius: 6px;
  padding: 8px 14px;
  cursor: pointer;
  font-size: 14px;
}
.nav-btn:hover {
  background: #f3f4f6;
}
.step-indicator {
  font-size: 14px;
  color: #4b5563;
}