function _ensureThemeRoot(sLibName, sThemeName) {
		var sThemeRoot = Theming.getThemeRoot(sThemeName, sLibName);
		if (sThemeRoot) {
			// check whether for this combination (theme+lib) a URL is registered or for this theme a default location is registered
			sThemeRoot = sThemeRoot + (sThemeRoot.slice( -1) == "/" ? "" : "/") + sLibName.replace(/\./g, "/") + "/themes/" + sThemeName + "/";
			LoaderExtensions.registerResourcePath((sLibName + ".themes." + sThemeName).replace(/\./g, "/"), sThemeRoot);
		}
	}
Makes sure to register the correct module path for the given library and theme
in case a themeRoot has been defined.

@param {string} sLibName Library name (dot separated)
@param {string} sThemeName Theme name
@private