Story

The current RH Theme is configured using `themes` config. As part of adopting Backstage’s New Frontend System, we need to migrate the plugin to use 
`ThemeBlueprint` or create extensions wherever needed.

Note: Only the app plugin can register/use ThemeBlueprint

Background

Dependencies and Blockers

QE impacted work

Documentation impacted work

Acceptance Criteria

• Design the Blueprint first and get approval from the team
• Themes are migrated to the New Frontend System model
• Existing Theme functionality must continue to work with no regression after the migration
• Should move the legacy app to app-legacy and the new app with NFS model should be in app

Refs:

NFS migration spike : https://docs.google.com/document/d/1Qb0T1Iu3ego50iO4V7NErSD44NdWSvk5n70pZvj-EmU/edit?tab=t.0#heading=h.zc5io9fn4km1

Backstage docs:
https://github.com/backstage/backstage/blob/master/docs/frontend-system/building-apps/08-migrating.md#themes