:root {
  --bg: #ffffff;
  --text: #1a1a1a;
  --muted: #6b7280;
  --border: #e5e7eb;
  --accent-start: #667eea;
  --accent-end: #764ba2;
  --card-bg: #f9fafb;
  --max-width: 760px;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg: #0f0f10;
    --text: #f3f4f6;
    --muted: #9ca3af;
    --border: #2a2a2e;
    --card-bg: #18181b;
  }
}

* {
  box-sizing: border-box;
}

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Helvetica Neue",
    "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", Arial, sans-serif;
  font-size: 16px;
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

header {
  background: linear-gradient(135deg, var(--accent-start) 0%, var(--accent-end) 100%);
  color: #ffffff;
  padding: 48px 20px 56px;
}

.header-inner {
  max-width: var(--max-width);
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  flex-wrap: wrap;
}

.brand {
  font-size: 28px;
  font-weight: 700;
  letter-spacing: -0.02em;
}

.lang-switch {
  display: inline-flex;
  background: rgba(255, 255, 255, 0.15);
  border-radius: 999px;
  padding: 4px;
  backdrop-filter: blur(8px);
}

.lang-switch button {
  background: transparent;
  border: none;
  color: #ffffff;
  padding: 6px 16px;
  border-radius: 999px;
  font-size: 14px;
  font-weight: 500;
  cursor: pointer;
  transition: background 0.15s ease;
}

.lang-switch button.active {
  background: rgba(255, 255, 255, 0.95);
  color: #4c3a8e;
}

.lang-switch button:not(.active):hover {
  background: rgba(255, 255, 255, 0.08);
}

.title-block {
  max-width: var(--max-width);
  margin: 28px auto 0;
}

h1 {
  font-size: 36px;
  margin: 0 0 8px;
  font-weight: 700;
  letter-spacing: -0.02em;
}

.subtitle {
  font-size: 15px;
  opacity: 0.9;
  margin: 0;
}

main {
  max-width: var(--max-width);
  margin: 0 auto;
  padding: 40px 20px 80px;
}

section {
  margin-bottom: 36px;
}

h2 {
  font-size: 22px;
  font-weight: 600;
  margin: 0 0 12px;
  letter-spacing: -0.01em;
  padding-bottom: 8px;
  border-bottom: 1px solid var(--border);
}

h3 {
  font-size: 17px;
  font-weight: 600;
  margin: 20px 0 8px;
}

p {
  margin: 0 0 12px;
}

ul {
  padding-left: 22px;
  margin: 0 0 12px;
}

li {
  margin-bottom: 6px;
}

a {
  color: var(--accent-start);
  text-decoration: none;
  border-bottom: 1px solid transparent;
  transition: border-color 0.15s ease;
  word-break: break-word;
}

a:hover {
  border-bottom-color: var(--accent-start);
}

.callout {
  background: var(--card-bg);
  border-left: 3px solid var(--accent-start);
  padding: 14px 18px;
  border-radius: 6px;
  margin: 16px 0;
  font-size: 15px;
}

.callout strong {
  color: var(--accent-end);
}

footer {
  border-top: 1px solid var(--border);
  padding: 32px 20px 48px;
  text-align: center;
  color: var(--muted);
  font-size: 14px;
}

footer .footer-inner {
  max-width: var(--max-width);
  margin: 0 auto;
}

footer p {
  margin: 4px 0;
}

[data-lang-en] [lang="zh"],
[data-lang-zh] [lang="en"] {
  display: none;
}

@media (max-width: 600px) {
  header {
    padding: 32px 18px 40px;
  }
  h1 {
    font-size: 28px;
  }
  h2 {
    font-size: 19px;
  }
  .brand {
    font-size: 24px;
  }
  main {
    padding: 28px 18px 60px;
  }
}