[EuiProvider] Provide data about parent and default theme configurations

[thompsongl]

Related: https://github.com/elastic/kibana-team/issues/805

EuiProvider ships with a default theme value for convenience, but the implicit fallback can cause problems when a component renders outside of the context tree. Missing a themeMode toggle is the most common, resulting in light/dark inconsistencies.

Similarly, inverting the colorMode does not always indicate a desire for the entire subcomponent tree to inherit. For instance, a design decision for all popovers to be light mode regardless of context.

Both can be addressed by adding more data about top-level EuiProvider use and configuration. That data can then be used to enforce usage by warn logging or throwing exceptions.

[github-actions]

👋 Hey there. This issue hasn't had any activity for 180 days. We'll automatically close it if that trend continues for another week. If you feel this issue is still valid and needs attention please let us know with a comment.

[github-actions]

❌ We're automatically closing this issue due to lack of activity. Please comment if you feel this was done in error.

[cee-chen]

This is still valid.

[github-actions]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.

[cee-chen]

IMO, various recent updates to EuiProvider and EuiThemeProvider resolve this issue. #6949 #6775