Improve hub index card UI for light and dark mode

- Add proper dark mode backgrounds with subtle accent-tinted gradients
  instead of `dark:bg-none` which left cards flat
- Use accent colors for headings in dark mode instead of generic gray
- Add bottom border separator between card header and links
- Unify link hover styles with accent-colored text across all cards
- Strengthen border visibility (200 in light, 800 in dark)
- Enhanced hover effects with shadow-md and border color transitions
- Differentiate Storage Buckets card with sky color palette
- Improve "new" badge dark mode styling

Made-with: Cursor

Author: enzostvs