Arrow styling

Hello,

I'm working on a fundraising page for a non-profit organization and am trying to turn the default arrow ("e62a" for down "e62d" for up) into a plus and a minus sign ("e60f" and "e613" respectively). The two images show how it is now and how I would like it to be.

In the font_icons.css the arrows are called .frm_arrowup6_icon the plus sign I want .frm_plus4_icon and the minus sign .frm_minus4_icon.

I've tried to change the content for the frm_arrow_icon class in my child theme which did not work. I've also tried changing the FrmStylesHelper.php since the code for the default arrow appears there (see below). But the arrows never change, even if I delete entire passages from the .css and .php files.

public static function icon_key_to_class( $key, $icon = '+', $type = 'arrow' ) {
if ( 'arrow' == $type && is_numeric($key) ) {
//frm_arrowup6_icon
$arrow = array( '-' => 'down', '+' => 'up' );
$class = 'frm_arrow' . $arrow[ $icon ];
} else {
//frm_minus1_icon
$key = str_replace('p', '', $key);
$plus = array( '-' => 'minus', '+' => 'plus' );
$class = 'frm_' . $plus[ $icon ];
}

if ( $key ) {
$class .= $key;
}
$class .= '_icon';

return $class;
}

Probably there's a simple solution for this which I'm too blind to see. Any help would be much appreciated!