Hi @stepan-phiture, and welcome to the Auth0 Community!
You are correct that with the New Universal Login, the theme applies to all the applications in the tenant. To achieve your use case, you will have to use the Classic Login and customize the branding of your pages. This Knowledge Article describes the process, including conditionally rendering the theme based on the client_id of the application.
I hope this helps you solve your use case!
Kind regards,
Teodor.