Fix doc_rust_generate.sh script execution and add theme-specific CSS selectors

Fixed critical bugs preventing script execution:
- Fixed verbose() function return code causing exit with set -e
- Fixed check_dependencies() missing return 0
- Removed spinner that was hanging with set -euo pipefail
- Added progress messages for cargo doc execution

Enhanced CSS theming:
- Wrapped CSS variables in :root[data-theme="dark"] selector
- Added :root[data-theme="light"] variant with inverted colors
- Higher specificity to override rustdoc default rules
- Light theme uses lighter backgrounds and darker text for proper contrast

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

Co-Authored-By: Claude <noreply@anthropic.com>

assets/doc_rust_generate/theme/header.html.jinja

@@ -9,7 +9,7 @@
       primaryTextColor: '{{primaryTextColor}}',
       primaryBorderColor: '{{primaryBorderColor}}',
       lineColor: '{{lineColor}}',
-      secondaryColor: '#{{secondaryColor}}',
+      secondaryColor: '{{secondaryColor}}',
       tertiaryColor: '{{secondaryColor}}',
       background: '{{background}}',
       mainBkg: '{{mainBkg}}',