public static void registerCustomDefaultsSource( String packageName ) {
		registerCustomDefaultsSource( packageName, null );
	}
Registers a package where FlatLaf searches for properties files with custom UI defaults.
<p>
This can be used to specify application specific UI defaults that override UI values
of existing themes or to define own UI values used in custom controls.
<p>
There may be multiple properties files in that package for multiple themes.
The properties file name must match the used theme class names.
E.g. {@code FlatLightLaf.properties} for class {@link FlatLightLaf}
or {@code FlatDarkLaf.properties} for class {@link FlatDarkLaf}.
{@code FlatLaf.properties} is loaded first for all themes.
<p>
These properties files are loaded after theme and addon properties files
and can therefore override all UI defaults.
<p>
Invoke this method before setting the look and feel.
<p>
If using Java modules, the package must be opened in {@code module-info.java}.
Otherwise, use {@link #registerCustomDefaultsSource(URL)}.

@param packageName a package name (e.g. "com.myapp.resources")
flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java