chore: remove styled-system and useTheme from BaseStyles

[francinelucca]

• Closes https://github.com/github/primer/issues/5429

Changelog

New

Changed

Removed

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• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
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Testing & Reviewing

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[siddharthkp]

Does this break anything in the AnchoredOverlay styles?

[francinelucca]

I don't see any usage of these props in the AnchoredOverlay module css file, this is just about the test file being wrapped in BaseStyles Im pretty sure

[siddharthkp]

Strange, didn't expect this story to update 🤔

[francinelucca]

removed!

[primer-integration]

🟢 ci completed with status success.