How can I change the appearance of login/register?

For the CSS values of the MFE you can fork the :

repository and installing it on the images using a hook plugin that looks sort of like this:

from tutor import hooks

hooks.Filters.ENV_PATCHES.add_item(
    (
        "mfe-dockerfile-post-npm-install",
        """
RUN npm install '@edx/brand@git+https://github.com/name of your fork.git'
"""
    )
)

Following that, build the mfe image and restart it or start -d to see your changes.

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