feat(portal): Add dark mode theme support #50
Description
Description
Add support for dark mode theme in the Portal with user preference selection.
Features
- Theme selector in user profile/settings
- Options: Light, Dark, and System (follow OS preference)
- Theme preference persisted between sessions
- Smooth transition when switching themes
Acceptance Criteria
- Theme toggle available in user settings
- All screens properly styled for both themes
- System preference option detects OS setting
- No flash of wrong theme on page load
- Preference saved and remembered on next login