style: comprehensive visual design improvements

- Add CSS variables for consistent theming across light/dark modes
- Improve typography: larger font sizes, better line heights, proper hierarchy
- Implement post card design with borders, shadows, and hover effects
- Enhance visual separation with improved spacing and decorative elements
- Add styled CTAs with animations
- Improve code blocks, blockquotes, tables, and image styling
- Add responsive design for mobile devices
- Include accessibility features: focus states, smooth scroll, selection styling

CSS expanded from 66 to 538 lines with organized sections.

Author: prabha-git