TYPO3 Core

Just so that you see how it could look like if one includes a custom CSS (HTML hacked in browser for the footer, no logo reused on that MFA form) but better consistency in the authentication workflow imho:

Custom CSS:

.card-login { background-color: inherit; }
.card-login .card-heading { background-color: rgba(255,255,255,.6); }
.card-login .card-body { background-color: rgba(255,255,255,.6); border-bottom-color: #db4f30; }
.card-login .card-footer { background-color: #fff; }
.typo3-login-logo { padding-top: 0; }

/* Needed until https://forge.typo3.org/issues/93898 is fixed: */
#t3-login-submit-section .btn-login { background-color: #db4f30; }
.typo3-login { background-image: url("/typo3conf/ext/my-ext/Resources/Public/Backend/Images/LoginBackground.jpg"); }

/* Unrelated to previous bug but better UX doing that: */
.card-login .card-heading h2.text-center { margin-top: .75rem; }

Patch set 1 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/68765

Hi Xavier, thanks for reporting this. You're completely right, MFA should also respect the backend extension configuration.
This however requires to move the current logic from the LoginController into a more general class, which can then also be used in e.g. the MfaController.
Since this is already quite a huge change, I would like to avoid further drive-by changes, so only fixing this bug, without introducing new features.

Nevertheless I would like to take a look at the further suggestions:

Reuse of other properties as well, such as the background image (background-image on CSS .typo3-login) and logo of the app so that the context stays the same

The logo is not used here since the main focus should be on the MFA provider to be used for the additional authentication step. Back then, I actually thought about adding
the MFA providers' logo here as well, but decided against to not unnecessarily overload this view.

Possibly add another "mfa" class on the .card-login block so that one can override some styles based on main authentication form or mfa/2fa form

This seems reasonable but should not be done in this patch, since it's more a feature than a bug.

What about keeping the copyright and TYPO3 logo with the card footer? Instead of "More about TYPO3", this could be "More about MFA/2FA", even if logically people reaching this step should know about it already.

The footer note is now (latest patches) also displayed at the right bottom of the screen. The copyright / TYPO3 logo in the .card-footer is not displayed, since this space is required for the
alternative authentication providers (see screenshot). I furthermore don't thinks this is needed for the additional step. For example, in case you are using mfa_webauthn, the view is only displayed
a few seconds - in case the security key is already inserted.

Are you fine with just fixing the bug (not respecting extension configuration) and adding another ticket for some further improvements of the authentication view?

Applied in changeset 23644892f4ef6cb00c3ebb32689591ad93593961.