Storybook: Add 'Jetpack Dashboard' background color

[retrofox]

This PR adds the Jetpack Dashboard background color, along with Light and Dark ones.

Changes proposed in this Pull Request:

• Storybook: Add 'Jetpack Dashboard' background color

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Run storybook

cd ./jetpack/projects/js-packages/storybook

> pnpm install & pnpm storybook:dev

• Open http://localhost:50240
• Take a look at the background colors

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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[renatoagds]

LGTM