viewer.addListener(SWT.PaintItem, this);

@Override
public void handleEvent( Event event) {
    switch (event.type) {
    case SWT.PaintItem:
        paintItemBorder(event);
        break;
    }

protected void paintItemBorder( Event event) {
        if (getLinesVisible()) {
            return;
        }

        // Draw line at bottom and right side
        int x1 = event.x;
        int y1 = event.y;

        int x2 = x1 + getColumnWidth(event.index) - 1;
        int y2 = y1 + getItemHeight() - 1;

        GC gc = event.gc;
            gc.setForeground(Display.getDefault().getSystemColor(SWT.COLOR_RED));
        gc.drawLine(x1, y2, x2, y2); //Change line width by manipulating these values
        gc.drawLine(x2, y1, x2, y2);
    }