How to remove the border on a JMenu or JMenuItem

Question

I'm using Eclipse Luna with Java 1.7 and am trying to create a JPanel that has a JMenuBar. This JMenuBar contains a JMenu which again contains a JMenuItem. To fit the whole JMenuBar into my project's visual design, I changed the background and foreground colors of JMenu, JMenuBar and JMenuItem using UIManager.put(). The result should be a drop-down menu with sections only divided by different shades of a Color (blue in my example). No lines or borders.

However, a whiteish, silverish etched border remains on the MenuItem.

My question is this: How can I make this border disappear?

I'd also like to know what element this border appears on and if it's a "feature" or there's some kind of sense to it.

SSCCE:

package sscce;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.UIManager;
@SuppressWarnings("serial")
public class GameUIPanel extends JFrame {
    public GameUIPanel() {
        setLayout(new BorderLayout());
        setPreferredSize(new Dimension(800, 600));
        setResizable(false);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        setLayout(new BorderLayout());
        setBackground(Color.decode("#00a2e8"));

        // create and adjust Components
        UIManager.put("MenuBar.background", Color.decode("#00a2e8"));
        UIManager.put("MenuBar.selectionBackground", Color.decode("#0092e4"));

        UIManager.put("Menu.background", Color.decode("#00beff"));
        UIManager.put("Menu.selectionBackground", Color.decode("#00a2e8"));
        UIManager.put("Menu.foreground", Color.decode("#91593c"));
        UIManager.put("Menu.selectionForeground", Color.decode("#7e3c1a"));

        UIManager.put("MenuItem.background", Color.decode("#00beff"));
        UIManager.put("MenuItem.selectionBackground", Color.decode("#00a2e8"));
        UIManager.put("MenuItem.foreground", Color.decode("#91593c"));
        UIManager.put("MenuItem.selectionForeground", Color.decode("#7e3c1a"));

        UIManager.put("PopupMenu.border",
                BorderFactory.createLineBorder(Color.decode("#00beff"), 1));
        UIManager.put("PopupMenu.foreground", Color.decode("#000000"));

        JMenuBar menubar = new JMenuBar();
        menubar.setBorder(BorderFactory.createLineBorder(
                Color.decode("#7e3c1a"), 2, false));

        JMenu gameMenu = new JMenu("Game");
        gameMenu.setOpaque(true);
        gameMenu.setFont(Font.decode("Arial-BOLD-24"));

        JMenuItem back = new JMenuItem("Back");
        back.setFont(Font.decode("Arial-BOLD-24"));

        gameMenu.add(back);
        menubar.add(gameMenu);

        setJMenuBar(menubar);
        pack();
        setVisible(true);
    }
    public static void main(String[] args) {
        GameUIPanel gui = new GameUIPanel();
    }
}

Added an image, as requestet. I hope this helps.