Como colocar o JInternalFrame com o mesmo LookAndFeel do JFrame?

package erp.athos;

import erp.aop.gui.MSG;
import erp.usuario.JanelaCadastroUsuario;
import java.awt.Container;
import java.awt.Dimension;
import javax.swing.JDesktopPane;
import javax.swing.JInternalFrame;
import javax.swing.JOptionPane;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.WindowConstants;

/**
 *
 * @author paulo
 */
public class JanelaAthos extends javax.swing.JFrame {

    private Container container;
    private JDesktopPane desktopPane;

    public JanelaAthos() {
        try {
            UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
        } catch (UnsupportedLookAndFeelException | InstantiationException | IllegalAccessException | ClassNotFoundException ex) {
            MSG.erroGeral(ex.toString());
        }        

        desktopPane = new JDesktopPane();
        container = getContentPane();
        container.add(desktopPane);
        initComponents();
        iniciarGUI();
    }

    private void iniciarGUI() {

        setTitle("Athos");
        setSize(990, 620);
        setPreferredSize(new Dimension(990, 620));
        setMinimumSize(new Dimension(990, 620));
        setDefaultCloseOperation(WindowConstants.DO_NOTHING_ON_CLOSE);
        setLocationRelativeTo(null);
        criarInternalFrame();
        SwingUtilities.updateComponentTreeUI(this);
    }
}