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. 
We could come up with an implementation of `ThemeSource` or perhaps `ResourceBundleThemeSource` that might be able to construct the theme, or change the `ServiceThemeResolver` to check for this. I wanted to avoid custom implementations as much as possible, but I can dive deeper and see what exactly may be involved. If it's fairly trivial then I agree that we should remove hardcoded values from the UI.