How can I set background to an Absolute Layout in Java?

Question

this.setBackground() doesn't seem to affect the background of the mainframe Jframe without removing setLayout(null) I set to it. How do I resolve this?
Here's my code:

public class MainFrame extends JFrame {

    public final static int SQUARE_UNIT = 35;
    int frameHeight = SQUARE_UNIT* 22;
    public final int frameWidth = SQUARE_UNIT *22;
    Color frameColor = new Color(0x20252B);

    MainFrame() {
        this.setTitle("Tetris");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setResizable(false);
        this.setPreferredSize(new Dimension(frameWidth,frameHeight));
        this.setLayout(null);
        this.setBackground(frameColor);
    }

    public static void main(String[] args) {

        MainFrame gameFrame = new MainFrame();
        Board tetrisBoard = new Board(MainFrame.SQUARE_UNIT,MainFrame.SQUARE_UNIT);
        gameFrame.add(tetrisBoard);

        gameFrame.setVisible(true);
        gameFrame.pack();
    }
}