WordPress ribosome example.

functions.php

<?php

/**
 * Child theme's function.php. This is loaded *before* the parent theme's so you can
 * unhook things here as you wish. See: https://github.com/WordPress/WordPress/blob/e9dbea9aac6300550f1637cda24948df65752a68/wp-settings.php#L366-L369
 * 
 * STYLESHEETPATH === child theme path
 * TEMPLATEPATH === parent theme path
 * 
 * For example, in Ribosome https://wordpress.org/themes/ribosome/ say you didn't like the way
 * it handled the custom header integration. Unhook it's version, hook in your own.
 */
 
 // priority, the last argument here, means your hook fires before ribosomes
 // which is important because....
 add_action('after_setup_theme', 'you_yourtheme_setup', 9);
 function you_yourtheme_setup()
 {
    // ... you need to unhook the ribosome custom header callback from `after_setup_theme`
    // it was hooked in at priotity 10, the default.
    remove_action('after_setup_theme', 'ribosome_custom_header_setup');
   
    add_theme_support('custom-header', [
      // your args...
    ]);
    
    /* Ribosome should have done this to make your life easier
    add_theme_support('custom-header', apply_filters('ribosome_custom_header_args', [
      // ...
    ]));
    
    So you could hook into it and change things.
    add_filter('ribosome_custome_header_args', function (array $args) {
      // change stuff
      return $args;
    });
    */
 }