Closes THE-34, THE-35, THE-36.
- Replaces all hardcoded colors in arf.css with CSS custom properties
- Dark palette defined on :root (default); light palette on body.light-mode
- Removes old .dark-Mode class; D3 inline styles now read CSS vars at render time
- goDark() toggles light-mode class and persists preference to localStorage
- Flash-prevention inline script in body applies stored theme before first paint
- Toggle button label reflects current theme state
Co-Authored-By: Paperclip <noreply@paperclip.ing>
