[test_operator] Add dark/light mode toggle to stackviz reports

Add theme switching functionality to stackviz HTML reports:
- CSS variables for complete dark/light theme support
- Toggle button in header to switch themes
- Theme preference saved to localStorage
- Smooth transitions between themes
- Canvas elements (timeline/minimap) update with theme
- All UI components (modals, tooltips, search) are theme-aware
- Defaults to dark mode

Users can now switch between dark and light modes based on their
preference, and the choice persists across page reloads.

Signed-off-by: sauragar <sauragar@redhat.com>

Author: sauragar