You don’t need to change the core authentication package — just keep its logic and rebuild the UI on top of it.
Most auth packages (like Firebase, NextAuth, Laravel Breeze, etc.) let you:
Use their backend/auth logic (login, signup, reset password).
Create your own custom forms and screens that call those same functions or endpoints.
Example (React + NextAuth):
signIn("credentials", { email, password });
You can style your own form and buttons however you want — the auth logic stays the same.