show "back" link for password connectors

This way, the user who has selected, say, "Log in with Email" can make up
their mind, and select a different connector instead.

However, if there's only one connector set up, none of this makes sense -- and
the link will thus not be displayed.

Signed-off-by: Stephan Renatus <srenatus@chef.io>

web/themes/coreos/styles.css

@@ -107,3 +107,7 @@
   text-align: left;
   width: 250px;
 }
+
+.theme-link-back {
+  margin-top: 4px;
+}