I have a web app that has a group of users in Azure AD and I\'d like to authenticate against that via username/password but customize the solution with my own styled login scree
Customizing login page in Azure AD (not B2C) is currently limited to Branding update. But that is available only in Premium tier.
You can upvote the feedback here.
Another option is Resource Owner Password Credentials with your own page but Microsoft recommends NOT to use ROPC flow. Also MFA would not work in ROPC.
When you said "The username/password "password" grant only works if MFA is enabled", not sure if I could follow that! Can you explain?
And, custom user flow is only limited to B2C.