[bootstrap/styles] add early support for v8 themes

[spalger]

Fixes #61123

Summary

Implements initial support for the version 8 themes the EUI team is working on. To switch between themes there is a theme:version advanced setting which currently has two possible values v7 and v8. Changing this value and reloading the page will load the v7 or v8 css styles from @kbn/ui-shared-deps and the v7 or v8 theme files will be exported from @elastic/eui/dist/eui_theme_light.json, @elastic/eui/dist/eui_theme_dark.json, as well as a new module @kbn/ui-shared-deps/eui_theme. This new module exports euiThemeDark and euiThemeLight which represent the two previously mentioned sets of theme vars, as well as euiTheme which is automatically mapped to the right theme based on the current value of the theme:* settings.

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[snide]

I did a quick run through this last night, and it worked as I'd expect. I don't think all the JSON import conversions needs to be done in this PR, but the method/example you set up makes sense to me and is clean.

Thanks for jumping on this so quickly, it's really gonna help us as we build out the new theme.

[spalger]

@elasticmachine merge upstream

[snide]

@spalger Is this reviewable?

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)

[spalger]

@elasticmachine merge upstream

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: e1d1345

History

• 💚 Build #48939 succeeded d656b16
• 💚 Build #48436 succeeded ce85e14
• 💔 Build #48207 failed d3a3ba8
• 💔 Build #48180 failed 441bff8

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[tylersmalley]

LGTM for Ops

[snide]

I've tested this in browser to make sure it works how I'd suspect. I also did a brief run through of the code to make sure it's flexible enough for designs needs. Likely there will be some downstream changes in the apps that will need to follow, but nothing with the current defaults will have any ripples from this change.

Thanks for adding this one so quickly. 🎉