apache
Remove support for `FAB APP_THEME`
Copied from #15277:
Resolves https://github.com/apache/airflow/issues/13158.
As the UI has evolved, it has needed to diverge from the constraints of the aged, secondary Bootstrap dependency (by way of FAB). The alternative Bootstrap themes provided by FAB are no longer working properly, resulting in a sub-par user experience for anyone who might opt to use one. Supporting these themes would require substantial ongoing maintenance overhead.
The alternative themes don't provide a better user experience than the (newly refreshed) UI theme. I could see differentiation between multiple Airflow deployments as the goal for utilizing this feature. In that case, we have the option to set a different nav bar color by setting a color value in the navbar_color config.
While this PR doesn't stop a user (who knows the inner workings of FAB) from configuring an alternative theme, it does ensure that the airflow theme is always loaded. There is no documentation of this feature in the Airflow docs, but this PR also removes comments suggesting usage of it and pointing to the FAB docs.
I've also updated the theme CSS file naming to remove "default" so it doesn't suggest that it is something changeable.
This needs to wait for Airflow 3.0.
