chore: forward @primer/react theming from @primer/styled-react under FF

[francinelucca]

Relates to https://github.com/github/primer/issues/6635

Changelog

New

• Added a new feature flag, primer_react_styled_react_use_primer_theme_providers, to DefaultFeatureFlags, enabling toggling between Primer's and styled-components' theming providers.
• Implemented FeatureFlaggedTheming.tsx, which exports ThemeProvider, useTheme, BaseStyles, and useColorSchemeVar components/functions that switch behavior based on the feature flag's state.
• Updated index.tsx to re-export theming types and components from FeatureFlaggedTheming instead of the previous locations, centralizing the feature-flagged logic.

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

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-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: 8894da7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@primer/react	Patch
@primer/styled-react	Patch

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[Copilot]

Pull request overview

This PR introduces a feature-flagged theming “forwarder” in @primer/styled-react so consumers can toggle between the legacy styled-components theming implementation and @primer/react’s theming providers.

Changes:

• Added primer_react_styled_react_use_primer_theme_providers to @primer/react default feature flags.
• Added FeatureFlaggedTheming.tsx in @primer/styled-react to switch ThemeProvider, useTheme, and BaseStyles based on the flag.
• Updated @primer/styled-react exports to route theming exports through the feature-flagged module, and added browser tests + changeset.

Show a summary per file

File	Description
packages/styled-react/src/index.tsx	Redirects theming exports (ThemeProvider/BaseStyles + types) to the new feature-flagged entrypoint.
packages/styled-react/src/components/FeatureFlaggedTheming.tsx	Implements the feature-flag switch between @primer/react and legacy styled-react theming APIs.
packages/styled-react/src/tests/FeatureFlaggedTheming.browser.test.tsx	Adds tests validating behavior when the feature flag is enabled vs disabled.
packages/react/src/FeatureFlags/DefaultFeatureFlags.ts	Registers the new feature flag default (false).
.changeset/fuzzy-kangaroos-cover.md	Declares patch releases for @primer/react and @primer/styled-react.

Copilot's findings

Comments suppressed due to low confidence (1)

packages/styled-react/src/components/FeatureFlaggedTheming.tsx:40

• useColorSchemeVar is always re-exported from the styled-react ThemeProvider. When the feature flag is enabled, ThemeProvider switches to @primer/react's ThemeProvider, so the styled-react theme context is no longer populated and useColorSchemeVar will always fall back (it reads colorScheme from the styled-react context default). This should be feature-flagged the same way as useTheme (e.g. delegate to @primer/react's useColorSchemeVar when enabled).

export {useColorSchemeVar}

• Files reviewed: 5/5 changed files
• Comments generated: 2

[primer]

🤖 Lint issues have been automatically fixed and committed to this PR.

[siddharthkp]

tiny nit, approving in advance

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/20206

[primer-integration]

Integration test results from github/github-ui:

  CI Failed
  VRT Passed
  Projects Passed

CI check runs linting, type checking, and unit tests. Check the workflow logs for specific failures.

Need help? If you believe this failure is unrelated to your changes, please reach out to the Primer team for assistance.