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: 45b46a7

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

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[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.

[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/19716

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!

[siddharthkp]

(nit) would this same changelog show up for both the packages?

I'd suggest splitting them or skipping the changelog for primer/react

[siddharthkp]

tiny nit, approving in advance