The Fedora Bubbles and Bluecurve themes for the gdm greeter have accessibility problems. More specifically, the username/password text field does not have the ATK "labelled by" relation making it difficult for a screen reader to announce the purpose of the text box to a blind user. The plain, unthemed greeter correctly provides this relation. But since picking an login theme requires admin priviledges, switching to the plain greeter is not a solution for unpriviledged users. Contact me if you need help using ATK to correct this problem.
On second look, this is problem probably exists not in the theme, but in the gdmgreeter application. I'll close this bug and push for a correction upstream at gnome.org.