:root {
            --chem-primary: #4A90E2; 
            --chem-ink: #0f172a; 
            --chem-line: #dbeafe; 
            --chem-soft: #eff6ff; 
            --chem-warm: #fff7ed; 
        }

        .bst-hero { background: linear-gradient(135deg, #eff6ff 0%, #ffffff 52%, #fff7ed 100%); border: 1px solid var(--chem-line); border-radius: 12px; padding: clamp(20px, 4vw, 36px); margin-bottom: 24px; }
        .bst-hero .eyebrow { color: var(--chem-primary); font-weight: 800; text-transform: uppercase; font-size: 0.82rem; }
        .bst-hero h1 { color: var(--chem-ink); line-height: 1.14; margin: 10px 0 12px; }
        .bst-hero p { color: #475569; line-height: 1.75; max-width: 820px; }

        .bst-facts { display: grid; grid-template-columns: repeat(auto-fit, minmax(170px, 1fr)); gap: 12px; margin: 22px 0; }
        .bst-fact { background: #ffffff; border: 1px solid #e5e7eb; border-radius: 8px; padding: 14px; }
        .bst-fact span { display: block; color: #64748b; font-size: 0.82rem; font-weight: 800; }
        .bst-fact strong { display: block; color: #111827; line-height: 1.35; margin-top: 6px; }

        .bst-toc { display: flex; flex-wrap: wrap; gap: 10px; margin: 18px 0 30px; }
        .bst-toc a { background: var(--chem-soft); border: 1px solid var(--chem-line); border-radius: 999px; color: var(--chem-primary); font-size: 0.9rem; font-weight: 800; padding: 8px 12px; text-decoration: none; }

        .bst-question { border: 1px solid #e5e7eb; border-left: 4px solid var(--chem-primary); border-radius: 8px; background: #ffffff; padding: 16px; margin: 18px 0; }
        .bst-answer { background: #f8fafc; border-radius: 8px; color: #334155; line-height: 1.75; margin-top: 12px; padding: 14px; }
        
        .bst-download-panel { align-items: center; background: #ffffff; border: 1px solid var(--chem-line); border-radius: 10px; display: flex; flex-wrap: wrap; gap: 14px; justify-content: space-between; margin: 18px 0 24px; padding: 16px; }
        .bst-coin-btn { background: var(--chem-primary); border: 0; border-radius: 8px; color: #ffffff; cursor: pointer; font-weight: 800; padding: 11px 16px; }
        .bst-coin-btn:hover { background: #357abd; }
        
        .bst-chapter-nav { display: grid; grid-template-columns: repeat(auto-fit, minmax(180px, 1fr)); gap: 10px; margin-top: 18px; }
        .bst-chapter-nav a { background: #ffffff; border: 1px solid #e5e7eb; border-radius: 8px; color: #1f2937; font-weight: 800; padding: 12px; text-decoration: none; }
        .bst-chapter-nav a.active, .bst-chapter-nav a:hover { background: var(--chem-soft); border-color: var(--chem-primary); color: var(--chem-primary); }

        .concept-list { padding-left: 20px; }
        .concept-list li { margin-bottom: 8px; }
        .word-table { width: 100%; border-collapse: collapse; margin-top: 10px; }
        .word-table th, .word-table td { border: 1px solid #e5e7eb; padding: 10px; text-align: left; }
        .word-table th { background-color: var(--chem-soft); color: var(--chem-ink); }

        body.dark-mode .bst-hero { background: linear-gradient(135deg, #111827 0%, #0f172a 56%, #1e293b 100%) !important; border-color: #334155 !important; }
        body.dark-mode .bst-hero h1, body.dark-mode .bst-fact strong { color: #f8fafc !important; }
        body.dark-mode .bst-hero p, body.dark-mode .bst-answer { color: #cbd5e1 !important; }
        body.dark-mode .bst-fact, body.dark-mode .bst-question, body.dark-mode .bst-answer, body.dark-mode .bst-download-panel, body.dark-mode .bst-chapter-nav a { background: rgba(17, 24, 39, 0.94) !important; border-color: #334155 !important; }
       .bst-hero { padding: 2rem 1.5rem; background-color: #f0f4ff; text-align: center; border-radius: 8px; margin-bottom: 2rem; }
        .bst-hero .eyebrow { font-size: 0.9rem; font-weight: bold; color: #4a69bd; text-transform: uppercase; }
        .bst-hero h1 { font-size: 2.5rem; margin: 0.5rem 0; color: #1e2a53; }
        .bst-hero p { max-width: 800px; margin: 1rem auto 0; color: #555; }
        .bst-facts { display: grid; grid-template-columns: repeat(auto-fit, minmax(250px, 1fr)); gap: 1rem; margin-bottom: 2rem; padding: 1.5rem; background-color: #f9f9f9; border-radius: 8px; }
        .bst-fact { background-color: #fff; padding: 1rem; border-radius: 6px; box-shadow: 0 2px 4px rgba(0,0,0,0.05); }
        .bst-fact span { display: block; font-size: 0.85rem; color: #666; margin-bottom: 0.25rem; }
        .bst-fact strong { font-size: 1rem; color: #333; }
        .bst-toc { display: flex; flex-wrap: wrap; gap: 0.75rem; background-color: #e9edff; padding: 1rem; border-radius: 8px; margin-bottom: 2rem; }
        .bst-toc a { text-decoration: none; background-color: #4a69bd; color: white; padding: 0.5rem 1rem; border-radius: 20px; font-size: 0.9rem; transition: background-color 0.3s; }
        .bst-toc a:hover { background-color: #3b528f; }
        .bst-question { background-color: #fff; border: 1px solid #e0e0e0; border-radius: 8px; padding: 1.5rem; margin-bottom: 1.5rem; box-shadow: 0 1px 3px rgba(0,0,0,0.04); }
        .bst-answer { background-color: #f7f9fc; border-left: 4px solid #4a69bd; padding: 1rem; margin-top: 1rem; border-radius: 0 4px 4px 0; }
        .bst-answer table { width: 100%; border-collapse: collapse; margin-top: 1rem; }
        .bst-answer th, .bst-answer td { border: 1px solid #ccc; padding: 0.8rem; text-align: left; }
        .bst-answer th { background-color: #e9edff; }
        .concept-list li { margin-bottom: 0.75rem; }
        .faq-accordion { margin-bottom: 1rem; border: 1px solid #ddd; border-radius: 5px; }
        .faq-accordion summary { font-weight: bold; padding: 1rem; cursor: pointer; background-color: #f9f9f9; }
        .faq-answer { padding: 1rem; background-color: #fff; border-top: 1px solid #ddd; }
        figure { margin: 1.5rem 0; text-align: center; }
        figure img { max-width: 100%; height: auto; border-radius: 8px; box-shadow: 0 4px 8px rgba(0,0,0,0.1); }
        figcaption { margin-top: 0.5rem; font-style: italic; color: #666; font-size: 0.9rem; }