Add dark/light theme toggle with auto-detect

[Nanle-code]

Description

Implement comprehensive theme system with OS-level dark mode detection.

Requirements

• Toggle between dark and light themes
• Auto-detect system preference
• Save theme preference
• Smooth theme transitions
• Accessible contrast ratios
• Support for custom themes

Acceptance Criteria

• Themes switch without lag
• Colors meet WCAG standards
• Preference persists
• Auto-detection works on all browsers