Update Primer React to v38

[danielguillan]

Towards https://github.com/github/primer/issues/6134

List of notable changes:

• Upgraded @primer/react to version 38 in all relevant package.json files.
• Remove usage of Box from @primer/react.
• Remove usage of the sx prop in favor of CSS modules and inline styles.

No visual changes expected.

[Copilot]

Pull request overview

This PR upgrades @primer/react from v37 to v38 across all packages in the monorepo, migrating away from Primer React's deprecated Box component and sx prop pattern in favor of CSS Modules and inline styles with CSS custom properties.

Key Changes:

• Upgraded @primer/react to v38.0.0 in package.json files (theme, site, root)
• Refactored components to remove Box usage and replace with native HTML elements (div, span)
• Migrated from sx prop to CSS Modules and inline styles using CSS custom properties
• Updated tests to remove dependency on Primer theme providers where no longer needed

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/theme/package.json	Upgrades @primer/react dependency to v38.0.0
packages/theme/components/layout/sidebar/Sidebar.tsx	Replaces sx prop with inline styles using CSS custom properties for margin spacing
packages/theme/components/layout/root-layout/Theme.tsx	Removes Box component usage, replaces with native div elements and inline styles; removes explanatory comments for breadcrumb filtering logic
packages/theme/components/layout/root-layout/Theme.module.css	Adds new CSS module with classes that are not currently imported or used in the component
packages/theme/components/layout/nav-drawer/Drawer.tsx	Migrates from Box with sx prop to motion.div with CSS module classes
packages/theme/components/layout/nav-drawer/Drawer.module.css	Adds CSS module for drawer overlay and panel styling
packages/theme/components/content/dos-and-donts/DosAndDonts.tsx	Removes Box component, replaces with native div and inline styles
packages/theme/components/content/caption/Caption.tsx	Refactors from Primer's Text component to native span with CSS module
packages/theme/components/content/caption/Caption.test.tsx	Updates tests to use standard render instead of renderWithPrimerThemeProviders
packages/theme/components/content/caption/Caption.module.css	Adds CSS module for caption styling
packages/site/package.json	Upgrades @primer/react dev dependency to v38.0.0
package.json	Upgrades @primer/react dev dependency to v38.0.0 in root
package-lock.json	Updates lockfile with new v38 dependencies and peer dependency changes

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[rezrah]

Looks good. Only some minor feedback about removal of inline styles, otherwise LGTM 🚢