To add your custom icons into the theme, first you should load the icons into the WordPress site, the icons may be loaded in different ways depending on the the icons that you are using, for example, by loading icons files manually via the child theme, or by using a third party scripts like fontawesome.
For example, fontawsome requires adding your icons kit’s code into the <Head> of the website, you can do this by pasting the code into Theme options > Layout > Advanced > Custom header tags include (or by manually adding it via the child theme).
After that you can add the custom icons using this filter code snippet into functions.php file in the child theme:
add_filter( 'pixfort_custom_font_icons', 'custom_icons_filter', 40, 1 );
function custom_icons_filter($icons) {
$new = array(
array( 'fas fa-thumbs-up' => 'fas fa-thumbs-up' ),
array( 'fas fa-ambulance' => 'fas fa-ambulance' ),
);
return array_merge( $icons, $new );
}
*You can add all the desired new icons list into the $new
array.
After that the new icons will be displayed into icons selector in the theme elements.