Background style tokens

## Summary
Provide a set of tokens used for default background colours. Both light and dark backgrounds need providing for either a light or a dark theme.

## 💬 Description
Currently there is no guidance on how to specify colours for backgrounds of pages, sections, overlays, and other content.

We need a set of default tokens for specifying backgrounds, and guidance on how best to use them.

We need a set of background colours so that layers can be built up within an application.

## 💰 User value
This provides a standard way of colouring backgrounds and helps to ensure accessible colour contrasts. They can also be used in theming to provide more customization for the user.

## 📚 User Stories
If relevant, describe the high-level functionality as user stories.

*As an* ICDS user:
*I need* a way to set background colour in my app
*So that* they are used consistently.

## 📝 Acceptance Criteria
If relevant, describe in full detail the different interactions and edge cases that the component or patterns needs to fulfil.

*Given*
*When*
*Then*

## ✏ Designs
If there's a Figma design file (or other mock-up), include it here.

## 🧾 Guidance
If there's written guidance or documentation, include a link to it here.

## Additional info
Tell us anything else useful to help us understand your suggestion.

The issue names no files, tests, entry points, designs, or guidance links. Start by clarifying the required background token set, light and dark theme behavior, layering, contrast expectations, and usage guidance; completion criteria are not yet specified.

Written by the indexing model from the issue text.