[Lens] Fix Treemap outer labels with transparent background

[dej611]

Summary

Fixes #83525

The outer layer has a transparent background, precisely a rgba(0,0,0,0) (note 0 opacity), which makes the elastic charts library produce a white text color. The library ignores the alpha parameter when computing the contrast color, so it requires a hint about dark mode.
This PR makes the component aware of the dark mode theme state and hints the chart library about the Kibana background color.
Tested on Lens + Dashboard

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[dej611]

Notice that the fix will not automatically work in Canvas:

The background color hint passed to the library does not cover Canvas page "background".

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 6ee33aa

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
lens	937.0KB	937.1KB	+172.0B

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@elasticmachine merge upstream

[mbondyra]

This PR fixes the severe bug, so I think it's a step in the right direction. Tested in Chrome, code LGTM

However, I don't like that we introduce another 'problem' in Canvas when Kibana is in the dark mode, but canvas has white background. In the future, with more customisability options maybe we'll allow users to fix it by using their own background color or font color, or we'll pass a background from canvas to set the proper color for the text.

[dej611]

Passing the Canvas Page background color to the embeddable would be great, in general I think, agree.
Also more customisation would help as well on this side.

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create backports run node scripts/backport --pr 84245 or prevent reminders by adding the backport:skip label.

[kibanamachine]

Looks like this PR has a backport PR but it still hasn't been merged. Please merge it ASAP to keep the branches relatively in sync.