我需要关于JavaSwingforGUI的帮助。我包括 frame.getcontentpane().setBackground(color.cyan); 但帧背景颜色不变。

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

public class LoginOne {

    private static JLabel lblUsr;
    private static JButton btnNext;
    private static JTextField txtUsr;
    private static JFrame frame;
    private static JPanel panel;

    public static void main(String[] args) {
        frame = new JFrame("Home Page");
        frame.setSize(800,600);
        frame.getContentPane().setBackground(Color.cyan);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        panel = new JPanel();
        frame.add(panel);

        panel.setLayout(null);

        lblUsr = new JLabel("Username");            //Username Label
        lblUsr.setBounds(10,20,80,25);        
        panel.add(lblUsr);                    

        txtUsr = new JTextField(20);         //Username input field
        txtUsr.setBounds(80,20,80,25);       
        panel.add(txtUsr);                   

        btnNext = new JButton("Next");
        btnNext.setBounds(80,90,80,25);
        panel.add(btnNext);

        frame.setVisible(true);
    }
}