refactor(targets): Organize CSS for targeting and add PostCSS transform

[zetlen]

Description

• Change all hardcoded colors in Venia CSS stylesheets to use CSS Variables instead.
• Add a postcss transform type to the transformModules target, to enable third-party manipulation of CSS.

Together these changes enable large-scale CSS transformations like the dark theme example shown here, which integrates with the code in this PR.

Acceptance

Verification Stakeholders

Community!

Verification Steps

Follow the walkthrough to test the color theme.

Checklist

• I have added tests to cover my changes, if necessary.

• I have updated the documentation accordingly, if necessary.

[PWAStudioBot]

	Fails
🚫	node` failed.
🚫	A version label is required. A maintainer must add one.
🚫	Unit tests in the following files did not pass 😔. All tests must pass before this PR can be merged packages/pwa-buildpack/lib/Utilities/__tests__/loadEnvironment.spec.js
🚫	No linked issue found. Please link a relevant open issue by adding the text "closes #<issue_number>" or "closes JIRA-<issue_number>" in your PR.

	Messages
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
📖	Access a deployed version of this PR here. Make sure to wait for the "pwa-pull-request-deploy" job to complete.

Log

ERROR ON TASK: unitTests

Error:  Danger had errors running. See message(s) above for more details.
danger-results://tmp/danger-results.json

Generated by 🚫 dangerJS against cb598ca

[devops-pwa-codebuild]

Performance Test Results

The following fails have been reported by WebpageTest. These numbers indicates a possible performance issue with the PR which requires further manual testing to validate.

https://pr-2460.pwa-venia.com : LH Performance Expected 0.85 Actual 0.57, LH Best Practices Expected 1 Actual 0.92
https://pr-2460.pwa-venia.com/venia-tops.html : LH Performance Expected 0.75 Actual 0.34, LH Best Practices Expected 1 Actual 0.92
https://pr-2460.pwa-venia.com/valeria-two-layer-tank.html : LH Performance Expected 0.8 Actual 0.48, LH Accessibility Expected 0.9 Actual 0.89, LH Best Practices Expected 1 Actual 0.92

[davemacaulay]

As there has been no movement on this PR in a significant amount of days we are going to close it, if you intend to continue work on this PR please comment and we can re-open it.