JTable how to remove header border

Well you were almost there...

For the header you can do:

final DefaultTableCellRenderer renderer = new DefaultTableCellRenderer();
renderer.setBorder(null);
header.setDefaultRenderer(renderer);

If you also want to remove the borders from the data cells also (practically removing the grid), then call:

table.setShowGrid(false);

Here is an assembled example of a JTable without any borders (except when the user selects a cell, then a border will appear around it temporarily):

import java.awt.Color;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.SwingUtilities;
import javax.swing.table.DefaultTableCellRenderer;
import javax.swing.table.JTableHeader;

public class Main {
    
    private static void createAndShowGUI() {
        final JFrame frame = new JFrame("Table without borders");
    
        final JTable table = new JTable(new Object[][]{
            new Object[]{"user", "user", "user"},
            new Object[]{"user", "user", "user"},
            new Object[]{"user", "user", "user"},
            new Object[]{"user", "user", "user"},
            new Object[]{"user", "user", "user"},
            new Object[]{"user", "user", "user"}
        }, new Object[]{"A", "B", "C"});
        
        JTableHeader header = table.getTableHeader();
        
        final Color dark = new Color(21, 25, 28),
                    orange = new Color(255, 117, 0);
        table.setForeground(orange);
        table.setBackground(dark);
        header.setBackground(dark);
        header.setForeground(orange);
        
        final DefaultTableCellRenderer renderer = new DefaultTableCellRenderer();
        renderer.setBorder(null); //Remove border from the renderer (which is just a JLabel)...

        //Apply the renderer to all header cells and all data cells:
        header.setDefaultRenderer(renderer);
        table.setDefaultRenderer(Object.class, renderer);
        
        /*The follwing lines simply remove the border from the cell editor
        (ie the component which draws each cell when the user edits its value):*/
        //final JTextField editor = new JTextField();
        //editor.setBorder(null);
        //table.setCellEditor(new DefaultCellEditor(editor));
        
        //header.setBorder(null); //Not needed.
        
        table.setShowGrid(false); //Hide the grid (which is some extra border between cells).
        
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(new JScrollPane(table));
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
    
    public static void main(final String[] args) {
        SwingUtilities.invokeLater(Main::createAndShowGUI);
    }
}