:root {
    --primary: #3498db;
    --secondary: #e67e22;
    --accent: #2ecc71;
    --dark: #2c3e50;
    --light: #ecf0f1;
    --text: #333;
    --white: #fff;
    --shadow: 0 4px 20px rgba(0,0,0,0.1);
    --radius: 12px;
    --transition: all 0.3s cubic-bezier(0.25, 0.8, 0.25, 1);
  }

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

  body {
    font-family: 'Roboto', sans-serif;
    line-height: 1.8;
    color: var(--text);
    background: linear-gradient(135deg, #f5f7fa 0%, #e4e8eb 100%);
    overflow-x: hidden;
  }

  .container {
    max-width: 1200px;
    margin: 0 auto;
    padding: 0 20px;
  }

  /* Header */
  .header {
    background: linear-gradient(135deg, var(--primary) 0%, #2980b9 100%);
    color: var(--white);
    padding: 60px 0;
    text-align: center;
    position: relative;
    overflow: hidden;
    border-radius: 0 0 40px 40px;
    box-shadow: var(--shadow);
    margin-bottom: 60px;
  }

  .header::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: url('https://images.gptkong.com/demo/sample1.png') no-repeat center right;
    background-size: contain;
    opacity: 0.1;
    pointer-events: none;
  }

  .header h1 {
    font-size: 3rem;
    margin-bottom: 20px;
    position: relative;
    z-index: 1;
    text-shadow: 0 2px 10px rgba(0,0,0,0.2);
  }

  .header p {
    font-size: 1.2rem;
    max-width: 700px;
    margin: 0 auto;
    position: relative;
    z-index: 1;
  }

  /* Navigation */
  .navbar {
    position: sticky;
    top: 0;
    background: rgba(255,255,255,0.95);
    backdrop-filter: blur(10px);
    padding: 15px 0;
    box-shadow: 0 2px 15px rgba(0,0,0,0.1);
    z-index: 1000;
  }

  .nav-container {
    display: flex;
    justify-content: space-between;
    align-items: center;
  }

  .nav-links {
    display: flex;
    gap: 30px;
  }

  .nav-links a {
    color: var(--dark);
    text-decoration: none;
    font-weight: 500;
    font-size: 1.1rem;
    transition: var(--transition);
    position: relative;
  }

  .nav-links a::after {
    content: '';
    position: absolute;
    bottom: -5px;
    left: 0;
    width: 0;
    height: 2px;
    background: var(--accent);
    transition: var(--transition);
  }

  .nav-links a:hover {
    color: var(--primary);
  }

  .nav-links a:hover::after {
    width: 100%;
  }

  .reference {
    background: var(--accent);
    color: var(--white);
    padding: 8px 15px;
    border-radius: 20px;
    font-size: 0.9rem;
    margin-left: 20px;
  }

  /* Article */
  article {
    background: var(--white);
    border-radius: var(--radius);
    padding: 50px;
    box-shadow: var(--shadow);
    margin-bottom: 60px;
  }

  article h2 {
    color: var(--primary);
    font-size: 2.2rem;
    margin-bottom: 30px;
    position: relative;
    padding-bottom: 15px;
  }

  article h2::after {
    content: '';
    position: absolute;
    bottom: 0;
    left: 0;
    width: 100px;
    height: 4px;
    background: linear-gradient(90deg, var(--primary), var(--accent));
    border-radius: 2px;
  }

  article h3 {
    color: var(--dark);
    font-size: 1.8rem;
    margin: 40px 0 20px;
    padding-left: 15px;
    border-left: 4px solid var(--secondary);
  }

  article p {
    font-size: 1.1rem;
    margin-bottom: 20px;
    color: #555;
  }

  /* Code blocks */
  pre {
    background: #282c34;
    border-radius: var(--radius);
    padding: 20px;
    margin: 25px 0;
    overflow: auto;
    box-shadow: var(--shadow);
  }

  code {
    font-family: 'Fira Code', monospace;
    color: #abb2bf;
    font-size: 0.95rem;
  }

  /* Images */
  .image-grid {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 25px;
    margin: 40px 0;
  }

  .image-card {
    border-radius: var(--radius);
    overflow: hidden;
    box-shadow: var(--shadow);
    transition: var(--transition);
    position: relative;
  }

  .image-card:hover {
    transform: translateY(-10px);
    box-shadow: 0 15px 30px rgba(0,0,0,0.15);
  }

  .image-card img {
    width: 100%;
    height: 100%;
    object-fit: cover;
    transition: var(--transition);
  }

  .image-card:hover img {
    transform: scale(1.05);
  }

  /* Cards */
  .card-container {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
    gap: 30px;
    margin: 50px 0;
  }

  .card {
    background: var(--white);
    border-radius: var(--radius);
    padding: 30px;
    box-shadow: var(--shadow);
    transition: var(--transition);
    border-top: 4px solid var(--primary);
  }

  .card:hover {
    transform: translateY(-10px);
    box-shadow: 0 15px 30px rgba(0,0,0,0.15);
  }

  .card h4 {
    color: var(--primary);
    margin-bottom: 15px;
    font-size: 1.4rem;
  }

  /* Footer */
  .footer {
    background: var(--dark);
    color: var(--white);
    padding: 60px 0 30px;
    text-align: center;
  }

  .footer p {
    margin-bottom: 30px;
  }

  .back-to-top {
    position: fixed;
    bottom: 30px;
    right: 30px;
    background: var(--secondary);
    color: var(--white);
    width: 50px;
    height: 50px;
    border-radius: 50%;
    display: flex;
    justify-content: center;
    align-items: center;
    cursor: pointer;
    transition: var(--transition);
    box-shadow: 0 4px 15px rgba(0,0,0,0.2);
    opacity: 0;
    visibility: hidden;
  }

  .back-to-top.show {
    opacity: 1;
    visibility: visible;
  }

  .back-to-top:hover {
    background: #d35400;
    transform: translateY(-5px);
  }

  /* Responsive */
  @media (max-width: 768px) {
    .header h1 {
      font-size: 2.2rem;
    }

    .nav-container {
      flex-direction: column;
    }

    .nav-links {
      margin-top: 15px;
      flex-wrap: wrap;
      justify-content: center;
    }

    article {
      padding: 30px;
    }

    article h2 {
      font-size: 1.8rem;
    }

    article h3 {
      font-size: 1.5rem;
    }
  }

  /* Animations */
  @keyframes fadeIn {
    from { opacity: 0; transform: translateY(20px); }
    to { opacity: 1; transform: translateY(0); }
  }

  .fade-in {
    animation: fadeIn 0.8s ease forwards;
  }

  .delay-1 { animation-delay: 0.2s; }
  .delay-2 { animation-delay: 0.4s; }
  .delay-3 { animation-delay: 0.6s; }