Implement Theme System for UI

## Overview
Add a theme system allowing users to customize the application appearance with light/dark modes and custom themes.

## Business Value
Theme customization improves user experience and accessibility. Dark mode reduces eye strain.

## Current State
- Single theme (light mode)
- No theme switching
- No customization options

## Subtasks
- [ ] Design theme architecture
- [ ] Implement theme provider
- [ ] Add light/dark mode toggle
- [ ] Create theme configuration
- [ ] Add theme persistence
- [ ] Implement theme switching UI
- [ ] Test theme accessibility

## Implementation Steps
1. Create theme configuration system
2. Implement theme context/provider
3. Add theme toggle component
4. Update all components for theme support
5. Add theme persistence

## Acceptance Criteria
- [ ] Light and dark themes are available
- [ ] Theme preference is persisted
- [ ] Theme switching is smooth
- [ ] All components support themes
- [ ] Themes meet contrast requirements
- [ ] Theme system is extensible

## Estimated Effort
- Hours: 12-16
- Complexity: Medium