Jetpack: resolve WPDS design tokens in legacy dashboard CSS

[CGastrell]

Part of #49285. Companion to #49345 (the runtime half).

Proposed changes

This is the build-time half of WPDS-token loading, scoped to the legacy _inc/ dashboard (Settings / Dashboard / Debugger).

• projects/plugins/jetpack/tools/postcss.config.js: add @wordpress/theme/design-tokens.css to the @csstools/postcss-global-data files list (alongside the existing calypso-color-schemes entry).
• projects/plugins/jetpack/package.json: add @wordpress/theme@0.13.0 as a direct dependency so require.resolve( '@wordpress/theme/design-tokens.css' ) (exposed via the package's exports map) resolves deterministically.

Why build-time here (and runtime elsewhere)

The _inc/ dashboard's postcss runs postcss-custom-properties with preserve: false, which inlines var(--wpds-*, #fallback) to its hex fallback at build time and strips the var(). So on the legacy dashboard, token-based colors (e.g. Hello Dolly) were baked to #87a6bc and a runtime :root{--wpds-*} stylesheet has nothing to bind to.

Feeding the tokens into postcss-global-data makes postcss-custom-properties inline them to the actual token value (#707070) instead of the fallback.

The obvious alternative — flipping preserve: true so vars survive to runtime — is not viable here: @automattic/calypso-color-schemes (732 generic vars like --color-text) is only build-time-inlined and not loaded at runtime, so preserving would leave every no-fallback calypso var undefined → broken. That's the exact "extremely generic vars" blocker called out in the existing postcss.config.js comment. The modernized admin-ui pages don't have this constraint (they preserve var() and load tokens at runtime), so they get the runtime stylesheet in #49345 instead.

No version skew: both this build-time source and #49345's runtime source come from @wordpress/theme@0.13.0.

Verification

Rebuilt _inc/build/admin.css:

	Before (trunk)	After
#87a6bc (Dolly fallback) occurrences	present	0
#707070 (token value) occurrences	0	6

Standalone postcss check of the config also confirms .dolly{ color: var(--wpds-color-fg-content-neutral-weak, #87a6bc) } → color:#707070, while a generic var(--color-text) stays inlined (unchanged) — i.e. no calypso regression.

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. pnpm jetpack build plugins/jetpack (the CSS/webpack step; an unrelated composer build-development step may fail locally).
2. grep -c 87a6bc projects/plugins/jetpack/_inc/build/admin.css → 0; grep -c 707070 … → non-zero.
3. On a connected site, hard-refresh Jetpack → Settings / Dashboard and the Debugger: the Hello Dolly strip reads #707070 (neutral gray) instead of the old bluish #87a6bc.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the change/dashboard-load-design-tokens branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack change/dashboard-load-design-tokens

bin/jetpack-downloader test jetpack-mu-wpcom-plugin change/dashboard-load-design-tokens

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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