Use default theme, if no theme is found for the service

Deals with https://github.com/Jasig/cas/issues/699

In the event that a theme is misconfigured for a service in registry, fall back onto the default CSS theme file and do not crash. 
is there a way we can detect the absence of the theme and just set the theme to "default" so that we don't have to basically add these values around that we might forget to update?