Postcss Fallbacks

[markacianfrani]

Brief Description

This PR implements a PostCSS plugin to automatically add fallback values for our CSS custom properties, allowing the astro.css to be optional when not using light theme

JIRA Link

Related Issue

General Notes

Motivation and Context

Issues and Limitations

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change

Checklist

• [ ] This PR adds or removes a Storybook story.
• [ ] I have added tests to cover my changes.
• [ ] Regressions are passing and/or failures are documented
• [ ] Changes have been checked in evergreen browsers

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: c28499982e3ffe17f12168f381138b6d71fe21e2

The changes in this PR will be included in the next version bump.

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

[netlify[bot]]

✅ Deploy Preview for astro-preview canceled.

Name	Link
🔨 Latest commit	c28499982e3ffe17f12168f381138b6d71fe21e2
🔍 Latest deploy log	https://app.netlify.com/sites/astro-preview/deploys/6463b99570d4290008e766ce

[netlify[bot]]

✅ Deploy Preview for astro-stencil canceled.

Name	Link
🔨 Latest commit	c28499982e3ffe17f12168f381138b6d71fe21e2
🔍 Latest deploy log	https://app.netlify.com/sites/astro-stencil/deploys/6463b9959bb6fa00070304fe