Replace expand / collapse icons

Is there a better way to override the expand / collapse chevron icons?

This works but it’s of course not a good solution (modifications are in bold ):

<?php

/**

* Create HTML list of nav menu items.

*

* @since 3.0.0

* @uses Walker

*/

class BellowsWalker extends Walker_Nav_Menu {

public $config_id = 'main';

public $submenu_toggle_icon_expand = 'plus';

public $submenu_toggle_icon_collapse = 'minus';