Add custom theming example and docs for Primer React

[joshblack]

There was a great discussion over in: https://github.com/primer/react/discussions/6078 that highlighted how custom themes have changed for Primer React as we've moved from styled-components to CSS Modules. This issue tracks adding support for the following:

• A custom theme example in our repo that shows how to customize the theme
• A note in our migration docs around theme customization
• A section in our docs for theme customization, similar to what we had for sx

[sunnyi101]

Thanks for submitting this issue! We’ll review it during monthly planning preparation to consider picking up based on current priorities.

[github-actions[bot]]

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