public static void registerCustomDefaultsSource( URL packageUrl ) {
		if( customDefaultsSources == null )
			customDefaultsSources = new ArrayList<>();
		customDefaultsSources.add( packageUrl );
	}
Registers a package where FlatLaf searches for properties files with custom UI defaults.
<p>
See {@link #registerCustomDefaultsSource(String)} for details.
<p>
This method is useful if using Java modules and the package containing the properties files
is not opened in {@code module-info.java}.
E.g. {@code FlatLaf.registerCustomDefaultsSource( MyApp.class.getResource( "/com/myapp/themes/" ) )}.

@param packageUrl a package URL
@since 2
flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java