/* ── CSS Variables ── */
        :root {
            --primary: #0b1a2a;
            --primary-light: #1a3450;
            --secondary: #1e88b3;
            --secondary-hover: #2a9fd6;
            --accent: #5fc3e4;
            --text-dark: #1a2634;
            --text-light: #f7f9fc;
            --bg-light: #f7f9fc;
            --bg-white: #ffffff;
            --card-shadow: 0 4px 20px rgba(0, 0, 0, 0.06);
            --card-hover-shadow: 0 8px 30px rgba(0, 0, 0, 0.12);
            --border-radius: 12px;
            --transition: all 0.25s ease;
            --font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
        }

        /* ── Reset & Base ── */
        *,
        *::before,
        *::after {
            box-sizing: border-box;
            margin: 0;
            padding: 0;
        }
        html {
            scroll-behavior: smooth;
        }
        body {
            font-family: var(--font-family);
            background: var(--bg-light);
            color: var(--text-dark);
            line-height: 1.6;
            padding: 0;
            margin: 0;
        }
        a {
            text-decoration: none;
            color: var(--secondary);
        }
        a:hover {
            color: var(--secondary-hover);
            text-decoration: underline;
        }
        img {
            max-width: 100%;
            height: auto;
            display: block;
        }
        .container {
            max-width: 1280px;
            margin: 0 auto;
            padding: 0 24px;
        }
        .section-title {
            font-size: 2rem;
            font-weight: 700;
            margin-bottom: 16px;
            color: var(--primary);
            letter-spacing: -0.02em;
        }
        .section-subtitle {
            font-size: 1.1rem;
            color: #4a5a6a;
            margin-bottom: 32px;
            max-width: 720px;
        }
        .text-center {
            text-align: center;
        }

        /* ── Buttons ── */
        .btn {
            display: inline-block;
            padding: 12px 28px;
            border-radius: 30px;
            font-weight: 600;
            font-size: 1rem;
            cursor: pointer;
            transition: var(--transition);
            border: none;
            text-align: center;
            background: var(--secondary);
            color: #fff;
            text-shadow: 2px 2px 2px #000;
        }
        .btn:hover {
            background: var(--secondary-hover);
            text-decoration: none;
            transform: translateY(-2px);
            box-shadow: 0 8px 20px rgba(30, 136, 179, 0.3);
        }
        .btn-outline {
            background: transparent;
            color: var(--secondary);
            border: 2px solid var(--secondary);
        }
        .btn-outline:hover {
            background: var(--secondary);
            color: #fff;
            border-color: var(--secondary);
        }
        .btn-primary {
            background: var(--secondary);
            color: #fff;
        }
        .btn-primary:hover {
            background: var(--secondary-hover);
        }
        .btn-sm {
            padding: 8px 16px;
            font-size: 0.85rem;
        }

        /* ── Header ── */
        .site-header {
            background: var(--primary);
            color: #fff;
            padding: 16px 0;
            position: sticky;
            top: 0;
            z-index: 1000;
            box-shadow: 0 2px 12px rgba(0, 0, 0, 0.15);
        }
        .site-header .container {
            display: flex;
            align-items: center;
            justify-content: space-between;
            flex-wrap: wrap;
            gap: 12px;
        }
        .logo {
            font-weight: 800;
            font-size: 1.25rem;
            letter-spacing: -0.5px;
            display: flex;
            align-items: center;
            gap: 8px;
            color: #fff;
        }
        .logo span {
            color: var(--accent);
        }
        .logo small {
            font-weight: 400;
            font-size: 0.7rem;
            opacity: 0.7;
            margin-left: 6px;
        }
        .nav-links {
            display: flex;
            gap: 20px;
            font-size: 0.9rem;
            font-weight: 500;
            align-items: center;
            flex-wrap: wrap;
        }
        .nav-links a {
            color: #d0e4f0;
            transition: color 0.2s;
        }
        .nav-links a:hover {
            color: #fff;
            text-decoration: none;
        }
        .nav-links .btn-nav {
            background: var(--secondary);
            color: #fff;
            padding: 6px 16px;
            border-radius: 30px;
            font-weight: 600;
            transition: background 0.2s;
        }
        .nav-links .btn-nav:hover {
            background: var(--secondary-hover);
            text-decoration: none;
        }

        /* Search form in header */
        .search-form {
            display: flex;
            align-items: center;
            background: rgba(255, 255, 255, 0.1);
            border-radius: 30px;
            padding: 4px 4px 4px 16px;
            border: 1px solid rgba(255, 255, 255, 0.15);
            transition: var(--transition);
        }
        .search-form:focus-within {
            border-color: var(--accent);
            background: rgba(255, 255, 255, 0.18);
        }
        .search-form input {
            background: transparent;
            border: none;
            padding: 8px 8px 8px 0;
            color: #fff;
            font-size: 0.9rem;
            outline: none;
            width: 180px;
            font-family: var(--font-family);
        }
        .search-form input::placeholder {
            color: rgba(255, 255, 255, 0.6);
        }
        .search-form button {
            background: var(--secondary);
            border: none;
            color: #fff;
            padding: 8px 16px;
            border-radius: 30px;
            cursor: pointer;
            font-size: 0.9rem;
            transition: var(--transition);
            font-weight: 600;
        }
        .search-form button:hover {
            background: var(--secondary-hover);
        }

        /* ── Hero ── */
        .hero {
            background: linear-gradient(145deg, var(--primary) 0%, var(--primary-light) 100%);
            color: #fff;
            padding: 80px 0 70px;
            border-bottom: 4px solid var(--secondary);
            position: relative;
            overflow: hidden;
        }
        .hero::after {
            content: "";
            position: absolute;
            right: -10%;
            top: -20%;
            width: 500px;
            height: 500px;
            background: radial-gradient(circle, rgba(30, 136, 179, 0.15) 0%, transparent 70%);
            border-radius: 50%;
            pointer-events: none;
        }
        .hero .container {
            position: relative;
            z-index: 1;
        }
        .hero h1 {
            font-size: 3rem;
            font-weight: 800;
            letter-spacing: -1px;
            line-height: 1.2;
            margin-bottom: 20px;
        }
        .hero h1 span {
            color: var(--accent);
        }
        .hero p {
            font-size: 1.2rem;
            opacity: 0.9;
            max-width: 720px;
            margin-bottom: 32px;
        }
        .hero-buttons {
            display: flex;
            flex-wrap: wrap;
            gap: 16px;
            
        }
        .hero-buttons .btn {
            padding: 14px 36px;
            font-size: 1.05rem;
            
        }
        .hero-buttons .btn-outline {
            border-color: rgba(255, 255, 255, 0.5);
            color: #fff;
        }
        .hero-buttons .btn-outline:hover {
            background: #cc7a00;            
            border-color: #fff;
        }

        /* ── Featured Categories ──color: var(--primary); */
        .categories-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
            gap: 24px;
            margin-top: 16px;
        }
        .category-card {
            background: var(--bg-white);
            border-radius: var(--border-radius);
            padding: 24px 20px;
            box-shadow: var(--card-shadow);
            transition: var(--transition);
            border: 1px solid #e8edf3;
            display: flex;
            flex-direction: column;
            align-items: flex-start;
        }
        .category-card:hover {
            transform: translateY(-4px);
            box-shadow: var(--card-hover-shadow);
        }
        .category-card .icon {
            font-size: 2rem;
            color: var(--secondary);
            margin-bottom: 12px;
        }
        .category-card h3 {
            font-size: 1.1rem;
            font-weight: 700;
            margin-bottom: 8px;
            color: var(--primary);
        }
        .category-card p {
            font-size: 0.9rem;
            color: #4a5a6a;
            margin-bottom: 12px;
            flex-grow: 1;
        }
        .category-card .meta {
            font-size: 0.8rem;
            color: #6a7a8a;
            margin-bottom: 12px;
            font-weight: 500;
        }
        .category-card .btn-sm {
            margin-top: auto;
        }

        /* ── Article Cards ── */
        .articles-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
            gap: 28px;
            margin-top: 16px;
        }
        .article-card {
            background: var(--bg-white);
            border-radius: var(--border-radius);
            overflow: hidden;
            box-shadow: var(--card-shadow);
            transition: var(--transition);
            border: 1px solid #e8edf3;
            display: flex;
            flex-direction: column;
        }
        .article-card:hover {
            transform: translateY(-4px);
            box-shadow: var(--card-hover-shadow);
        }
        .article-card .img-placeholder {
            height: 160px;
            background: #d5e3ee;
            display: flex;
            align-items: center;
            justify-content: center;
            color: #4a5a6a;
            font-size: 0.9rem;
            font-weight: 500;
            background-size: cover;
            background-position: center;
            background-color: #e8edf3;
        }
        .article-card .content {
            padding: 20px 20px 24px;
            flex-grow: 1;
            display: flex;
            flex-direction: column;
        }
        .article-card .content .badge {
            display: inline-block;
            background: var(--secondary);
            color: #fff;
            font-size: 0.7rem;
            font-weight: 600;
            padding: 2px 12px;
            border-radius: 20px;
            letter-spacing: 0.3px;
            align-self: flex-start;
            margin-bottom: 8px;
        }
        .article-card .content h3 {
            font-size: 1.1rem;
            font-weight: 700;
            margin-bottom: 8px;
            color: var(--primary);
        }
        .article-card .content p {
            font-size: 0.9rem;
            color: #4a5a6a;
            margin-bottom: 12px;
            flex-grow: 1;
        }
        .article-card .content .meta {
            font-size: 0.8rem;
            color: #6a7a8a;
            display: flex;
            gap: 16px;
            flex-wrap: wrap;
            margin-bottom: 12px;
            border-top: 1px solid #eef3f8;
            padding-top: 12px;
        }
        .article-card .content .meta span {
            display: flex;
            align-items: center;
            gap: 4px;
        }
        .article-card .content .btn-sm {
            align-self: flex-start;
        }

        /* ── Calculator Cards ── */
        .calculators-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
            gap: 24px;
            margin-top: 16px;
        }
        .calculator-card {
            background: var(--bg-white);
            border-radius: var(--border-radius);
            padding: 24px 20px;
            box-shadow: var(--card-shadow);
            transition: var(--transition);
            border: 1px solid #e8edf3;
            text-align: center;
            display: flex;
            flex-direction: column;
            align-items: center;
        }
        .calculator-card:hover {
            transform: translateY(-4px);
            box-shadow: var(--card-hover-shadow);
        }
        .calculator-card .icon {
            font-size: 2.2rem;
            color: var(--secondary);
            margin-bottom: 12px;
        }
        .calculator-card h4 {
            font-size: 1rem;
            font-weight: 700;
            margin-bottom: 8px;
            color: var(--primary);
        }
        .calculator-card p {
            font-size: 0.85rem;
            color: #4a5a6a;
            margin-bottom: 16px;
            flex-grow: 1;
        }

        /* ── Learning Paths ── */
        .paths-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
            gap: 28px;
            margin-top: 16px;
        }
        .path-card {
            background: var(--bg-white);
            border-radius: var(--border-radius);
            padding: 24px 24px 28px;
            box-shadow: var(--card-shadow);
            border: 1px solid #e8edf3;
        }
        .path-card h4 {
            font-size: 1.2rem;
            font-weight: 700;
            color: var(--primary);
            margin-bottom: 12px;
            display: flex;
            align-items: center;
            gap: 8px;
        }
        .path-card ul {
            list-style: none;
            padding: 0;
            margin: 0 0 16px 0;
        }
        .path-card ul li {
            padding: 6px 0;
            border-bottom: 1px solid #f0f4f8;
            font-size: 0.9rem;
        }
        .path-card ul li:last-child {
            border-bottom: none;
        }
        .path-card ul li a {
            color: var(--primary);
            font-weight: 500;
        }
        .path-card ul li a:hover {
            color: var(--secondary);
            text-decoration: none;
        }

        /* ── Formula & Standards Library ── */
        .library-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
            gap: 20px;
            margin-top: 16px;
        }
        .library-item {
            background: var(--bg-white);
            border-radius: var(--border-radius);
            padding: 16px 18px;
            box-shadow: var(--card-shadow);
            border: 1px solid #e8edf3;
            transition: var(--transition);
        }
        .library-item:hover {
            transform: translateY(-2px);
            box-shadow: var(--card-hover-shadow);
        }
        .library-item h5 {
            font-size: 1rem;
            font-weight: 600;
            color: var(--primary);
            margin-bottom: 4px;
        }
        .library-item p {
            font-size: 0.85rem;
            color: #4a5a6a;
            margin-bottom: 4px;
        }
        .library-item .badge-sm {
            font-size: 0.7rem;
            background: #eef3f8;
            padding: 2px 10px;
            border-radius: 12px;
            color: #4a5a6a;
            display: inline-block;
            margin-top: 4px;
        }

        /* ── Workshop Tips ── */
        .tips-grid {
            display: grid;
            grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
            gap: 20px;
            margin-top: 16px;
        }
        .tip-item {
            background: var(--bg-white);
            border-radius: var(--border-radius);
            padding: 18px 20px;
            box-shadow: var(--card-shadow);
            border: 1px solid #e8edf3;
            display: flex;
            align-items: flex-start;
            gap: 12px;
            transition: var(--transition);
        }
        .tip-item:hover {
            transform: translateY(-2px);
            box-shadow: var(--card-hover-shadow);
        }
        .tip-item .icon {
            font-size: 1.5rem;
            color: var(--secondary);
            flex-shrink: 0;
            margin-top: 2px;
        }
        .tip-item .content h5 {
            font-size: 0.95rem;
            font-weight: 600;
            color: var(--primary);
            margin-bottom: 2px;
        }
        .tip-item .content p {
            font-size: 0.85rem;
            color: #4a5a6a;
        }

        /* ── Sections spacing ── */
        section {
            padding: 60px 0;
        }
        section:not(:last-child) {
            border-bottom: 1px solid #eef3f8;
        }
        .section-header {
            margin-bottom: 32px;
        }

        /* ── Footer ── */
        .site-footer {
            background: var(--primary);
            color: #b0c8d8;
            padding: 48px 0 32px;
            border-top: 4px solid var(--secondary);
            margin-top: 20px;
        }
        .site-footer .container {
            display: grid;
            grid-template-columns: 2fr 1fr 1fr;
            gap: 32px;
        }
        .site-footer h5 {
            color: #fff;
            font-size: 1rem;
            margin-bottom: 12px;
        }
        .site-footer a {
            color: #b0c8d8;
        }
        .site-footer a:hover {
            color: #fff;
            text-decoration: none;
        }
        .site-footer ul {
            list-style: none;
            padding: 0;
        }
        .site-footer ul li {
            padding: 4px 0;
        }
        .footer-bottom {
            grid-column: 1 / -1;
            border-top: 1px solid rgba(255, 255, 255, 0.08);
            padding-top: 20px;
            margin-top: 12px;
            font-size: 0.85rem;
            text-align: center;
        }

        /* ── Back to top ── */
        .back-to-top {
            position: fixed;
            bottom: 30px;
            right: 30px;
            background: var(--secondary);
            color: #fff;
            width: 48px;
            height: 48px;
            border-radius: 50%;
            display: flex;
            align-items: center;
            justify-content: center;
            font-size: 1.2rem;
            box-shadow: 0 4px 16px rgba(0, 0, 0, 0.2);
            transition: var(--transition);
            z-index: 999;
            border: none;
            cursor: pointer;
            opacity: 0;
            visibility: hidden;
            transform: translateY(20px);
        }
        .back-to-top.visible {
            opacity: 1;
            visibility: visible;
            transform: translateY(0);
        }
        .back-to-top:hover {
            background: var(--secondary-hover);
            transform: translateY(-4px);
        }

        /* ── Responsive ── */
        @media (max-width: 1024px) {
            .hero h1 {
                font-size: 2.4rem;
            }
            .categories-grid {
                grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
            }
        }
        @media (max-width: 768px) {
            .site-header .container {
                flex-direction: column;
                align-items: stretch;
            }
            .nav-links {
                justify-content: center;
                gap: 12px;
            }
            .search-form {
                width: 100%;
                margin-top: 8px;
            }
            .search-form input {
                width: 100%;
            }
            .hero {
                padding: 50px 0;
            }
            .hero h1 {
                font-size: 2rem;
            }
            .hero p {
                font-size: 1rem;
            }
            .hero-buttons .btn {
                padding: 12px 24px;
                font-size: 0.95rem;
            }
            .articles-grid {
                grid-template-columns: 1fr;
            }
            .categories-grid {
                grid-template-columns: repeat(auto-fill, minmax(160px, 1fr));
            }
            .calculators-grid {
                grid-template-columns: repeat(auto-fill, minmax(160px, 1fr));
            }
            .paths-grid {
                grid-template-columns: 1fr;
            }
            .site-footer .container {
                grid-template-columns: 1fr;
                gap: 24px;
            }
        }
        @media (max-width: 480px) {
            .hero h1 {
                font-size: 1.6rem;
            }
            .section-title {
                font-size: 1.6rem;
            }
            .categories-grid {
                grid-template-columns: 1fr 1fr;
            }
            .calculators-grid {
                grid-template-columns: 1fr 1fr;
            }
            .library-grid {
                grid-template-columns: 1fr 1fr;
            }
            .tips-grid {
                grid-template-columns: 1fr;
            }
        }

        /* ── Image placeholder styling ── */
        .img-placeholder {
            background: #d5e3ee;
            display: flex;
            align-items: center;
            justify-content: center;
            color: #4a5a6a;
            font-weight: 500;
            font-size: 0.9rem;
            background-size: cover;
            background-position: center;
            background-color: #e8edf3;
        }
        .img-placeholder i {
            font-size: 2rem;
            opacity: 0.5;
        }
        .img-placeholder .label {
            margin-left: 8px;
        }

        /* ── Misc ── */
        .mt-16 {
            margin-top: 16px;
        }
        .mb-8 {
            margin-bottom: 8px;
        }
        .flex-center {
            display: flex;
            align-items: center;
            justify-content: center;
        }
        .gap-8 {
            gap: 8px;
        }