Add comprehensive ChatGPT-style document theme optimized for modern interactive content:
**Theme Features:**
- Inter font family for clean, modern sans-serif typography
- Compact 580px width for chat-like reading experience
- High contrast (#1f1f1f text on white background)
- ChatGPT signature green (#10a37f) accent color
- Tight 1.5 line height for efficient information density
- Modern 8px border radius for contemporary feel
- Optimized code block styling with proper monospace fonts
**Technical Implementation:**
- Added 'chatgpt' theme to LAYERED_THEMES system (document scope)
- Full backward compatibility with TEMPLATE_STYLES and LEGACY_THEME_MAPPING
- CLI integration: `markitect md-render --theme chatgpt`
- Proper theme layering support (combines with light/dark modes)
**Quality Assurance:**
- Comprehensive 9-test suite covering all functionality (9/9 passing)
- Verified HTML generation and CSS styling
- Tested CLI integration and theme combinations
- Full compatibility with existing theme architecture
Successfully closes Issue #165 with compact, readable layout optimized for
interactive content following ChatGPT's interface design principles.
🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>
tegwick
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