Extend login body class to support custom background

[toscho]

It would be useful to have a class custom-background on the body element for wp-login.php. Then we could just write:

add_action( 'login_head', '_custom_background_cb' );

Currently we have to copy the whole function with a very tiny difference: replace body.custom-background with body.login. Not very elegant.

[toscho]

Add conditional ' custom-background' to login body class.

[nacin]

The login_body_class filter added in 3.5 makes this moot.