This interface allows for implementations based on session,
cookies, etc. The default implementation is FixedThemeResolver,
simply using a configured default theme.
Note that this resolver is only responsible for determining the
current theme name. The Theme instance for the resolved theme name
gets looked up by DispatcherServlet via the respective ThemeSource,
i.e. the current WebApplicationContext.
Use RequestContext.getTheme() to retrieve the current theme in
controllers or views, independent of the actual resolution strategy.