PostCSS: add WPDS design token fallbacks

[simison]

Proposed changes

• Adds PostCSS configs for injecting WPDS token fallbacks:
  • var( --wpds-color-background-surface-neutral ) → var( --wpds-color-background-surface-neutral, #fcfcfc )
• Flips preserve: true setting for postcss-custom-properties plugin, which changes how variables are renderered in final bundle. This is required for WPDS token usage, but conflicts with how @automattic/calypso-color-schemes was configured before which dates to times when var() support in browsers was lacking. See old comment for context.
  • Source: background: var( --wpds-color-bg-surface-neutral-strong );
  • In bundle before: background: #fff;
  • In bundle after: background: var( --wpds-color-bg-surface-neutral-strong, #fff );
• Ensure @automattic/calypso-color-schemes gets loaded in the frontend at runtime (see the above point)

WP Build already has fallbacks configured, so parts of the repo built with it are already fine with adding fallbacks.

Separate follow-up PRs coming up for:

• adding Stylelint, which will lint files to be without fallbacks, since they're now added at build time (Stylelint: lint wordpress/theme design token usage #50110),
• centralising PostCSS configs under projects/js-packages/webpack-config/src/postcss.js (WIP).

See build config and stylelint plugin docs for @wordpress/theme for details.

Related product discussion/links

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

Testing instructions

• Lint is passing
• Tokens get fallbacks in final bundles. A good way to test is with the search package:
  • jetpack build packages/search
  • These styles:
    

    jetpack/projects/packages/search/src/dashboard/components/experience-selector/experience-option.scss

    Lines 3 to 9 in a1548bf

    	.jp-search-experience-option {
    	position: relative;
    	padding: var(--wpds-dimension-padding-2xl);
    	background: var(--wpds-color-bg-surface-neutral-strong);
    	border: var(--wpds-border-width-xs) solid var(--wpds-color-stroke-surface-neutral);
    	border-radius: var(--wpds-border-radius-lg);
    	transition: border-color 120ms ease, box-shadow 120ms ease;

  • Now look like this at projects/packages/search/build/dashboard/jp-search-dashboard.css:

    .jp-search-experience-option {
    	position: relative;
    	padding: 24px;
    	padding: var(--wpds-dimension-padding-2xl, 24px);
    	background: #fff;
    	background: var(--wpds-color-bg-surface-neutral-strong, #fff);
    	border: 1px solid #dbdbdb;
    	border: var(--wpds-border-width-xs, 1px) solid var(--wpds-color-stroke-surface-neutral, #dbdbdb);
    	border-radius: 8px;
    	border-radius: var(--wpds-border-radius-lg, 8px);
    	transition: border-color 120ms ease, box-shadow 120ms ease;
    }

• Spot check other bundles as well

[github-actions]

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[github-actions]

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• 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 update/wpds-fallback-values branch.
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[jp-launch-control]

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. 🤷

Full summary · PHP report · JS report

[anomiex]

This makes minimal changes to the built artifacts: the only thing I see is in several places some minimized code that (un-minified) looks something like

body.jetpack_page_jetpack-something .jp-admin-page-tabs {
    background: var(--wpds-color-bg-surface-neutral-strong);
    border-bottom: var(--wpds-border-width-xs) solid var(--wpds-color-stroke-surface-neutral-weak);

gets changed to

body.jetpack_page_jetpack-something .jp-admin-page-tabs {
    background: #fff;
    border-bottom: 1px solid #f0f0f0;

which is presumably exactly the intent. 👍

Flips preserve: true setting for postcss-custom-properties plugin, which changes how variables are renderered in final bundle. This is required for WPDS token usage, but conflicts with how @automattic/calypso-color-schemes was configured before

I'm not seeing any changes in the output that seem related to this. 🤷

Ensure @automattic/calypso-color-schemes gets loaded in the frontend at runtime (see the above point)

Or this.

[anomiex]

I wonder why pnpm sometimes does this for certain people.