Ability to pick a default backgroundColor for story previews

### Clear and concise description of the problem

I don't see a way to configure the default background color for story previews.

### Suggested solution

Adding a property for default story preview backgrounds in the configuration would be great.

### Alternative

_No response_

### Additional context

_No response_

### Validations

- [X] Follow our [Code of Conduct](https://github.com/histoire-dev/histoire/blob/main/CODE_OF_CONDUCT.md)
- [X] Read the [Contributing Guidelines](https://github.com/histoire-dev/histoire/blob/main/CONTRIBUTING.md).
- [X] Read the [docs](https://histoire.dev/guide/).
- [X] Check that there isn't [already an issue](https://github.com/histoire-dev/histoire/issues) that requests the same feature to avoid creating a duplicate.

The issue asks for a configuration property controlling the default background color of story previews. Start by locating the story preview configuration and rendering entry points; done means users can set a default background color through configuration and previews use it when no per-story value is provided.

Written by the indexing model from the issue text.