You know, we talked about this some time before in this forum, and I’m not sure that we came up with a valid solution.
Basically, the default login screen is a WordPress file, not a Graphene one, so doing anything to it would require hacking into WP core files–not recommended. But I believe there is at least one plugin that will put a login form on your blog pages somewhere. (It’s also possible that we just discussed this as a possibility, however.) I would search around in the plugin repository to see if there is one available.