:root {
  --primary-color: #4f46e5;
  --text-color: #374151;
  --bg-color: #f3f4f6;
  --card-bg: #ffffff;
  --heading-color: #111827;
  --border-color: #e5e7eb;
  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.1), 0 2px 4px -1px rgba(0, 0, 0, 0.06);
  --font-family: 'Inter', system-ui, -apple-system, sans-serif;
}

@media (prefers-color-scheme: dark) {
  :root {
    --text-color: #d1d5db;
    --bg-color: #111827;
    --card-bg: #1f2937;
    --heading-color: #f9fafb;
    --border-color: #374151;
  }
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: var(--font-family);
  background-color: var(--bg-color);
  color: var(--text-color);
  line-height: 1.6;
  display: flex;
  justify-content: center;
  min-height: 100vh;
  padding: 2rem 1rem;
}

.container {
  background-color: var(--card-bg);
  width: 100%;
  max-width: 800px;
  border-radius: 1rem;
  box-shadow: var(--shadow-md);
  padding: 2rem 3rem;
  border: 1px solid var(--border-color);
}

header {
  border-bottom: 2px solid var(--border-color);
  padding-bottom: 1.5rem;
  margin-bottom: 2rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 1rem;
}

h1 {
  color: var(--heading-color);
  font-size: 1.75rem;
  font-weight: 700;
  letter-spacing: -0.025em;
}

.language-switcher {
  display: flex;
  gap: 0.5rem;
  background: var(--bg-color);
  padding: 0.25rem;
  border-radius: 0.5rem;
}

.lang-btn {
  background: none;
  border: none;
  padding: 0.5rem 1rem;
  border-radius: 0.375rem;
  cursor: pointer;
  font-weight: 500;
  color: var(--text-color);
  transition: all 0.2s ease;
  font-size: 0.875rem;
}

.lang-btn:hover {
  background-color: rgba(0, 0, 0, 0.05);
}

.lang-btn.active {
  background-color: var(--primary-color);
  color: white;
  box-shadow: var(--shadow-sm);
}

h2 {
  color: var(--heading-color);
  font-size: 1.25rem;
  margin-top: 2rem;
  margin-bottom: 1rem;
}

p, ul {
  margin-bottom: 1rem;
}

ul {
  padding-left: 1.5rem;
}

li {
  margin-bottom: 0.5rem;
}

footer {
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border-color);
  font-size: 0.875rem;
  color: var(--text-color);
  opacity: 0.8;
  text-align: center;
}

/* Language Visibility */
[data-lang="en"], [data-lang="vi"] {
  display: none;
}

body.lang-en [data-lang="en"] {
  display: block;
}

body.lang-vi [data-lang="vi"] {
  display: block;
}

/* Mobile Responsiveness */
@media (max-width: 640px) {
  .container {
    padding: 1.5rem;
  }
  
  header {
    flex-direction: column;
    align-items: flex-start;
  }
  
  .language-switcher {
    width: 100%;
  }
  
  .lang-btn {
    flex: 1;
    text-align: center;
  }
}