import javax.swing.plaf.basic.BasicComboBoxRenderer;

static void setComboBoxListFont(JComboBox cb, final Font font) {
  cb.setRenderer(new BasicComboBoxRenderer {
    new DefaultListCellRenderer defaultRenderer;

    public Component getListCellRendererComponent(JList list, Object value, int index, boolean isSelected, boolean cellHasFocus) {
      JLabel renderer = cast defaultRenderer.getListCellRendererComponent(list, value, index, isSelected,cellHasFocus);
      renderer.setFont(theFont);
      ret renderer;
    }
  });
}