Hi @ignacio.aba,
Ah yes, in this situation, you will need to update your branding theme instead of creating a new one.
To do so, please call the Mangaement API’s Update branding theme endpoint.
Lastly, you may find this related knowledge solution relevant and helpful:
https://community.auth0.com/t/custom-universal-login-themes-for-applications-on-the-one-tenant/129578
Cheers!
Rueben