@bubazoo
Long and the short of it: No, you cannot modify the login/register pages using a theme. The only way to modify them is by hacking the wp-admin.css file. I suppose this is because these pages are considered part of the admin interface which was never intended to be themed. So, for now, if you want to modify the look of these pages, you must hack your wp-admin.css file and go through all the trouble of keeping up with changes as they are introduced and all that.