UI Components: Add theme support for Toolbar

[karthikeya-io]

What?

Part of #69646

This PR introduces theming support for Toolbar component

Why?

Previously the Toolbar component lacks theming support and hard to read particularly in the dark-mode.

How?

Testing Instructions

Run storybook locally. (npm run storybook:dev)
Navigate to the Toolbar component.
Change the theme and confirm that there are no visibility issues.

Testing Instructions for Keyboard

Same

Screenshots or screencast

Default theme

Before	After

Dark theme

Before	After

[github-actions]

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[github-actions]

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[github-actions]

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[ciampo]

Let's wait for design feedback on that look should the toolbar have on dark themes (cc @WordPress/gutenberg-design )

[t-hamano]

@karthikeya-io Thanks for the PR!

Sorry, a similar PR (#69278) has already been submitted, so let me close this PR.

However, we appreciate your work!