:root {
  --bg: #ffffff;
  --bg-2: #f5f5f7;
  --text: #1d1d1f;
  --muted: #6e6e73;
  --line: #d2d2d7;
  --accent: #0071e3;
  --accent-text: #ffffff;
  --card: #ffffff;
  --radius: 18px;
  color-scheme: light dark;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #000000;
    --bg-2: #161617;
    --text: #f5f5f7;
    --muted: #a1a1a6;
    --line: #38383a;
    --accent: #2997ff;
    --card: #1c1c1e;
  }
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font: 17px/1.55 -apple-system, BlinkMacSystemFont, "SF Pro Text", "Helvetica Neue", "Segoe UI", Roboto, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
}
a { color: var(--accent); text-decoration: none; }
a:hover { text-decoration: underline; }

.nav {
  position: sticky; top: 0; z-index: 10;
  background: color-mix(in srgb, var(--bg) 82%, transparent);
  backdrop-filter: saturate(180%) blur(20px);
  -webkit-backdrop-filter: saturate(180%) blur(20px);
  border-bottom: 1px solid var(--line);
}
.nav-inner {
  max-width: 980px; margin: 0 auto; padding: 0 16px;
  height: 52px; display: flex; align-items: center; gap: 20px;
}
.brand { font-weight: 600; color: var(--text); letter-spacing: -0.01em; display: flex; align-items: center; gap: 8px; }
.brand:hover { text-decoration: none; }
.brand-mark { width: 24px; height: 24px; border-radius: 6px; background: linear-gradient(135deg, #2f7cf6, #1a4fd0); display: inline-grid; place-items: center; color: #fff; font-size: 13px; font-weight: 700; }
.nav-links { display: flex; gap: 18px; margin-left: auto; font-size: 14px; flex-wrap: wrap; }
.nav-links a { color: var(--muted); }
.nav-links a[aria-current="page"] { color: var(--text); }
.lang-toggle {
  border: 1px solid var(--line); background: transparent; color: var(--text);
  border-radius: 999px; padding: 4px 12px; font: inherit; font-size: 13px; cursor: pointer;
}

main { max-width: 760px; margin: 0 auto; padding: 48px 16px 80px; }
.hero { text-align: center; padding: 56px 16px 24px; max-width: 820px; margin: 0 auto; }
.hero h1 { font-size: clamp(36px, 7vw, 64px); line-height: 1.05; letter-spacing: -0.03em; margin: 0 0 16px; }
.hero p.lead { font-size: clamp(19px, 2.6vw, 24px); color: var(--muted); margin: 0 auto 28px; max-width: 640px; }
.btn {
  display: inline-block; background: var(--accent); color: var(--accent-text);
  border-radius: 999px; padding: 12px 22px; font-weight: 500;
}
.btn:hover { text-decoration: none; filter: brightness(1.08); }
.btn.secondary { background: transparent; color: var(--accent); }

.grid { display: grid; grid-template-columns: repeat(auto-fit, minmax(220px, 1fr)); gap: 16px; max-width: 980px; margin: 32px auto 0; padding: 0 16px; }
.card { background: var(--bg-2); border-radius: var(--radius); padding: 24px; }
.card h3 { margin: 0 0 6px; font-size: 19px; letter-spacing: -0.01em; }
.card p { margin: 0; color: var(--muted); font-size: 15px; }

h1 { font-size: clamp(30px, 5vw, 44px); letter-spacing: -0.025em; line-height: 1.1; margin: 0 0 8px; }
h2 { font-size: 22px; letter-spacing: -0.01em; margin: 40px 0 8px; }
h3 { font-size: 18px; margin: 24px 0 6px; }
.meta { color: var(--muted); font-size: 15px; margin: 0 0 32px; }
p, li { color: var(--text); }
ul { padding-left: 22px; }
li { margin: 6px 0; }
.callout { background: var(--bg-2); border-radius: var(--radius); padding: 20px 22px; margin: 24px 0; }
.callout p { margin: 0; }
kbd {
  font: 600 14px ui-monospace, SFMono-Regular, Menlo, monospace;
  border: 1px solid var(--line); border-bottom-width: 2px; border-radius: 6px; padding: 1px 6px; background: var(--card);
}
details { border-bottom: 1px solid var(--line); padding: 14px 0; }
details summary { cursor: pointer; font-weight: 600; list-style: none; display: flex; justify-content: space-between; gap: 12px; }
details summary::after { content: "+"; color: var(--muted); font-weight: 400; }
details[open] summary::after { content: "−"; }
details summary::-webkit-details-marker { display: none; }
details > *:not(summary) { color: var(--muted); }

footer { border-top: 1px solid var(--line); color: var(--muted); font-size: 13px; }
footer .inner { max-width: 980px; margin: 0 auto; padding: 20px 16px 40px; display: flex; flex-wrap: wrap; gap: 8px 20px; }
footer a { color: var(--muted); }

[data-lang="en"] .vi, [data-lang="vi"] .en { display: none !important; }