I just discovered that when I lock down the server to use by using the skinning service it does not use this server info when using the Create Account page. Instead it forces the new user to type in a server name.
Not every Newbie gets it, that he has to type in the greyed out server name of the underlying main login page!
My personal feeling is it should also lock this setting or at least pre-populate the server to use in the account setup page so the user just has to enter the account name/password.
Tested agains Spark 2.0.8 Skinning built on Jan 22.