docs(storybook): update theme and switch theme icon

[joshblack]

Closes #9825

This PR fixes our contrast issues with the two global toolbar items by reverting our theme changes (we would need to revisit them anyways for v11 tokens) and replaces the theme switcher icon with the paintbrush asset from Storybook

Changelog

New

Changed

• Revert theme to default storybook colors for contrast issues
• Update icon to paintbrush

Removed

Testing / Reviewing

• Verify the "toolbar icons" render correctly and have a visible style

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[abbeyhrt]

I can actually see the buttons now! Haha looks great to me!