This might be a dumb question but are the login part of WordPress considered to be part of the backend?
These pages are accessible for users who are neither identified nor authenticated, so they are part of the frontend.
By default, they look a little bit like the backend style. That's just because that's the only style WordPress "knows" by default, so this is done for consistency, not to indicate that they actually are the backend.
The small overlay that pops up when your session expires while you are using the backend is a special case and indeed part of the backend. It is currently built of the regular login page, so you have to exclude that case in your customizations.