public interface ThemeSource
Themes. This enables parameterization and internationalization of messages for a given 'theme'.
The returned Theme will resolve theme-specific messages, codes, file paths, etc (e.g. CSS and image files in a web environment).
themeName- the name of the theme
nullif none defined. Note that, by convention, a ThemeSource should at least be able to return a default Theme for the default theme name "theme" but may also return default Themes for other theme names.