Form01.java

import javax.swing.*;
import java.awt.*;

public class Form01 extends JFrame {
    private final static Color SKYDARK =new Color(128,200,255);

    private Form01() {
        initComponents();
    }

    private void initComponents() {
        JPanel panel = new JPanel();
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        final JMenuBar jMenuBar = new JMenuBar();
        ButtonGroup themeButtonGroup = new ButtonGroup();
        jMenuBar.setBackground(new Color(204, 255, 204));

        JMenu themeMenu = new JMenu("Оформление");

        JRadioButtonMenuItem themeSelector = new JRadioButtonMenuItem("Sky", true);
        themeButtonGroup.add(themeSelector);
        themeSelector.addActionListener((evt) -> jMenuBar.setBackground(SKYDARK));
        themeMenu.add(themeSelector);

        themeSelector = new JRadioButtonMenuItem("Ignis", false);
        themeButtonGroup.add(themeSelector);
        themeSelector.addActionListener((evt) -> jMenuBar.setBackground(Color.BLACK));
        themeMenu.add(themeSelector);

        JMenu settingsMenu = new JMenu("Настройки");
        settingsMenu.add(themeMenu);
        jMenuBar.add(settingsMenu);

        setJMenuBar(jMenuBar);

        panel.setPreferredSize(new Dimension(300, 300));
        getContentPane().add(panel);

        pack();
    }

    public static void main(String args[]) {
        try {
            for (UIManager.LookAndFeelInfo info : UIManager.getInstalledLookAndFeels()) {
                if ("Windows".equals(info.getName())) {
                    UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (Exception ex) {
            java.util.logging.Logger.getLogger(Form01.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        EventQueue.invokeLater(() -> new Form01().setVisible(true));
    }
}