[FEAT]: add light/dark background toggle for component demo previews

[FreshmanJ]

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Many demos are currently presented on dark backgrounds, which works well for dark-themed websites. However, developers building light-themed websites may also want to preview how components look in a light context.

I would like to propose adding a small light/dark background toggle to the component demo preview area.

Proposed behavior

• Keep the current dark background as the default.
• Add a Light / Dark toggle for demo preview containers.
• Only change the preview background, not the component source code.
• Avoid changing existing component APIs.
• Optionally persist the selected preview background in localStorage.
• For components that do not adapt well to a light background, the toggle can be hidden or disabled, keeping the current dark-only preview.

Why this is useful

This would help developers building light-themed websites evaluate components in a more realistic context without manually editing the demo code.

I would be happy to open a PR for this if the direction sounds good.

I have already tried this approach locally with several existing components, and many of them appear to work well with both dark and light preview backgrounds.
examples:

Validations

• I have checked other issues to see if my issue was already discussed or addressed