Set alternative Layout/Template/Partial path individually to use your own Fluid templates. There are some TypoScript constants which you can simply override:
templateRootPath = EXT:your_key/Resources/Private/Templates/
partialRootPath = EXT:your_key/Resources/Private/Partials/
layoutRootPath = EXT:your_key/Resources/Private/Layouts/
You have also the option to use your own template files for the backend login. Just adapt the following TypoScript constants:
layoutPath = EXT:your_key/Resources/Private/Layouts/
templatePath = EXT:your_key/Resources/Private/Templates/
settings.stylesheet = EXT:your_key/Resources/Public/Styles/Backend.css
Please make also sure that you configure the role mapping from Auth0 roles to TYPO3 user groups. Maybe you also want to set the admin flag for backend users, depending on an Auth0 role mapping.
It is possible to append additional parameters to the Auth0 login URL. For example, you can preselect a specific connection or open the registration tab (instead of the login tab). This can be implemented by the following TypoScript setup:
# key = value
login_hint = You will log in to our shop system.
connection = google-oauth2