feat: dark mode scaffolding

[briangregoryholmes]

• Add feature flag darkMode
• Refine theme palette generation
• Remove transition-colors class
• Remove explicit border-gray-200 classes
• Cleanup tailwind config
• Convert bg-white usage to bg-surface
• Add node script to generate colors.css file with necessary mappings
• Remove RillTheme component
• Upgrade chroma-js
• Add dark variation to code editor
• Create new theme Tailwind color, falling back to primary when not set

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[notion-workspace]

Dark Mode

[ericpgreen2]

Generally looking good! I started a UXQA doc here with a couple initial observations.