Task: Implement Light to Dark Mode Translation for Cards

**Summary**
Update the design foundations so card background colours translate correctly from light mode to dark mode, instead of requiring manual overrides in components.

[Card Example](https://www.figma.com/design/VhZHw3RudFNMzfPEEYchE9/UI-Kit---Foundations-Delivery?node-id=4291-17049&t=vR9b044Pz06y1uR0-4)
[Colour Foundations ](https://www.figma.com/design/VhZHw3RudFNMzfPEEYchE9/UI-Kit---Foundations-Delivery?node-id=503-3145&t=vR9b044Pz06y1uR0-4)

**Context**
Design clarified that:
- White / non-coloured cards should map to a specific gray treatment in dark mode.
- Coloured cards should use a different dark mode variant.
- The current foundations do not reflect the intended gray-tone mapping, so teams are manually adjusting colours in components.

**Problem**
The current light-to-dark translation behaviour for card backgrounds is not encoded at the foundation level. As a result:
- component implementations require manual colour overrides
- dark mode behaviour is inconsistent across cards

**Task**
Implement the light > dark mode translation in foundations for card background colours so components can inherit the correct mapping by default.

**Scope**
- Update foundation-level colour mappings for card backgrounds.
- Ensure white / non-coloured cards use the approved gray translation in dark mode.
- Ensure coloured cards use the approved dark mode variant.
- Remove the need for manual per-component overrides where possible.

**Acceptance criteria**
- Card background colours follow the approved light-to-dark mapping from foundations.
- White / non-coloured cards render with the correct gray treatment in dark mode.
- Coloured cards render with the correct dark mode variant.

**Notes / Risks**
- Although the change sounds simple, it may have broader consequences because it could affect all coloured cards.
- QA: please verify all coloured card surfaces in dark mode (e.g join card, event card, and postcards etc.) have the correct gray colour