https://java-swing-tips.blogspot.com/2011/04/transparent-jtableheader.html

TransparentHeaderTest.java

import java.awt.*;
import java.util.Objects;
import javax.swing.*;
import javax.swing.border.*;
import javax.swing.table.*;

public class TransparentHeaderTest {
  public JComponent makeUI() {
    Color alphaZero = new Color(0x0, true);

    String[] columnNames = {"AAA", "BBBB", "CCCCC"};
    Object[][] data = {
      {"aaa", "eee", "fff"}, {"bbb", "ggg", "hh"},
      {"CCC", "ii", "jj"}, {"DDD", "k", "llllll"}
    };
    JTable table = new JTable(new DefaultTableModel(data, columnNames)) {
      @Override public void updateUI() {
        super.updateUI();
        setOpaque(false);
        setFocusable(false);

        setRowSelectionAllowed(true);
        setFillsViewportHeight(true);
        setShowVerticalLines(false);
        setIntercellSpacing(new Dimension(0, 1));
        setRowHeight(24);

        setBackground(alphaZero);
        setSelectionForeground(getForeground());
        setSelectionBackground(new Color(0, 0, 100, 50));

        getTableHeader().setDefaultRenderer(new TransparentHeader());
        getTableHeader().setOpaque(false);
        getTableHeader().setBackground(alphaZero);
      }
    };
    JScrollPane scroll = new JScrollPane(table);
    scroll.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));

    return scroll;
  }
  public static void main(String... args) {
    EventQueue.invokeLater(() -> {
      JFrame f = new JFrame();
      f.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
      f.getContentPane().add(new TransparentHeaderTest().makeUI());
      f.setSize(320, 240);
      f.setLocationRelativeTo(null);
      f.setVisible(true);
    });
  }
}

class TransparentHeader implements TableCellRenderer {
  private final JLabel label = new JLabel();
  private final Border b = BorderFactory.createMatteBorder(0, 0, 1, 0, Color.BLACK);
  @Override public Component getTableCellRendererComponent(JTable table, Object value, boolean isSelected, boolean hasFocus, int row, int column) {
    label.setText(Objects.toString(value, ""));
    label.setHorizontalAlignment(SwingConstants.LEFT);
    label.setBorder(b);
    return label;
  }
}