Hi there,
Thank you for reaching out to us!
Reading through your message, I wanted to try and provide some information, as it could help others as well.
You should be able to customize the Universal Login using both the Management API and CLI, depending on your use-case. For details on using the Management API, I recommend reading through our documentation on how to Customize Universal Login Page Templates and also check out the following Community Post for a Code snippet example.
Regarding CLI, please check out the following Knowledge Article that details how to Deploy Customized Universal Login Page with Auth0 Deploy CLI.
Hope this helped!
Gerald