[WIP] Emotion & Propagate

[thompsongl]

Summary

A very much draft version of integrating emotion-js into EUI by way of a custom cascading property library ("Propagate", for now) built for theming.

Contents:

• emotion dependencies
• babel build updates
• incomplete and hack-ridden conversion of EUI theme files to .ts
• mostly complete conversion of Guide* components in src-docs to use emotion+propagate
• example conversion of EuiMark to emotion
• demo of runtime theme switching
• demo of runtime theme token value mutability (euiColorLightShade)

gatsby-eui-starter testing ground: https://github.com/thompsongl/eui-emotion-gatsby
yarn build && npm pack, then use the .tgz as the EUI target in this repo.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation
• Checked Code Sandbox works for the any docs examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3760/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3760/

[thompsongl]

Test failures due to missing theme context in the jest environment. Not detrimental at the moment.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3760/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3760/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3760/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3760/

[cchaos]

My official/final 😂 review and exploration of this POC and possible next steps are all documented here: https://app.simplenote.com/publish/D9dZkc

The important bits are the top (explaining what we aim to gain from CSS in JS) and the bottom (my proposal for moving forward) sections.

[thompsongl]

I'm going to close this in favor of making smaller, more targeted PRs, perhaps even cherry-picking commits from this branch. We can decide when the first context PR is ready if we want to make use of a merge-base feature branch.

Follow #3912 for updates

[cchaos]

Makes sense to me!