Webmin theme for login page doesn't switch (back) to "Authentic Theme" unless you also reset the theme (again) from the Webmin -> Configuration -> Themes -> Current Theme option.
In other words: the login form stays ugly, if you switch to "Gray Framed Theme" and then switch back to "Authentic Theme".
Webmin/Change Language and Theme module can override global settings. Make sure that Authentic Theme is also set there or defaul option is active.