style(demos): Fix remaining hardcoded colors in demos 11, 13, 15

Complete the Eliza-style theming for demos that had hardcoded colors:

Demo 11 (Analogies):
- Replaced hardcoded colors (#333, white, #e0e0e0, #f8f9fa, #667eea)
  with CSS variables (--text-primary, --surface-color, --border-color,
  --bg-secondary, --primary-color)
- Added demo-specific CSS variables for shadows/transitions

Demo 13 (BERT MLM):
- Major CSS overhaul replacing ~30 hardcoded colors with variables
- Added hero-header styling matching Eliza pattern
- Preserved semantic accent colors (orange for masked tokens)
- Added dark theme override for masked token visibility

Demo 15 (Chatbot Evolution):
- Fixed HTML to use hero-header structure
- Replaced ~25 hardcoded colors with CSS variables
- Added input field theming for dark mode support
- Preserved semantic accent colors (emotion bars, button hovers)

Demos 12 and 14 were already correctly themed - no changes needed.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: jeremymanning