Inherit core theme in React plugins

[pierrejeambrun]

Remove some unnecessary code so React Plugins can directly inherit the Chakra Theme for Airflow Core without duplicating or copy pasting any core theme.

Now in local development, we use Chakra default theme (black).

When loaded into airflow, nothing changes, theme is smooth and consistent:

Also importantly this fixes an issue where the 'plugin theme' when loaded would override the core theme. (which is not what we want). This was an issue for instance if I customized my core theme via the api.theme config option, then loading the plugin would restore back the Airflow Core UI default theme that was hard copy/pasted into plugins.

Now with a custom core theme, the UI and plugins are correctly customized:

[bbovenzi]

Good call!

[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

Status	Branch	Result
❌	v3-1-test

You can attempt to backport this manually by running:

cherry_picker 3e57f1e v3-1-test

This should apply the commit to the v3-1-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

[pierrejeambrun]

Manual backport #60405

[jscheffl]

If I apply this change on Edge provider plugin, does this break compatibility with Airflow <3.1.6? Or is it backwards-compatible?

[pierrejeambrun]

This is backward compatible. (I mean nothing breaks):

• Old provider on new core -> working, provider will read it's local theme which is the one from airflow core
• new provider on old core -> provider will try to look at the core provided theme, not finding it and default to its local theme (which is the chakra UI default one, which will look a bit different). If we want to make that full compatible we need to keep the full theme around. But I think it's a lot of work to maintain for really small gain.

PR here for edge provider:
#60417