¿Cómo agrego configuraciones a la página de opciones de fondo?

Estoy desarrollando un tema y quería agregar contenido adicional a la página de opciones de fondo incorporada. Sé cómo usar la API de configuración para crear nuevas opciones de tema y configuraciones, pero parece que no puedo entender dónde está realmente llamando a la página de fondo. He buscado wp-includes/theme.phpy todavía no hay mucho, excepto llamadas a las funciones de fondo y nada que realmente muestre la página.

¿No podría hacerse esto con una acción?

El contenido de la página para fondos personalizados se crea en wp-admin/custom-background.php. No hay ninguna acción disponible donde se imprimen los campos.

Para agregar nuevos campos tenemos que imprimir JavaScript en el pie de página. La acción es 'admin_footer-appearance_page_custom-background'.

Para guardar esos valores de campo tenemos que conectarnos 'load-appearance_page_custom-background'.

El siguiente ejemplo agrega una nueva opción para background-origin: útil si ha establecido un borde alrededor bodyen su hoja de estilo.

add_action(
    'load-appearance_page_custom-background',
    array ( 'T5_Background_Origin', 'get_instance' )
);

/**
 * Add a new row to the background options table for 'background-origin'.
 *
 * @author  Thomas Scholz http://toscho.de
 * @version 2012.09.10
 */
class T5_Background_Origin
{
    /**
     * Main instance.
     * @type object|NULL
     */
    protected static $instance = NULL;

    /**
     * The name for the option. Will be saved as theme option.
     *
     * @link http://www.w3.org/TR/css3-background/#the-background-origin
     * @type string
     */
    protected $option = 'background_origin';

    /**
     * Label on the left side of our new option.
     *
     * @type string
     */
    protected $table_header = 'Background Origin';

    /**
     * Return an instance.
     *
     * @wp-hook load-appearance_page_custom-background
     * @return object
     */
    public static function get_instance()
    {
        NULL === self::$instance and self::$instance = new self;
        return self::$instance;
    }

    /**
     * Save our option and register the form.
     *
     * @wp-hook load-appearance_page_custom-background
     */
    public function __construct()
    {
        add_action(
            'admin_footer-appearance_page_custom-background',
            array ( $this, 'form' )
        );

        if ( empty ( $_POST[ $this->option ] ) )
        {
            return;
        }

        check_admin_referer( $this->option, "_t5nonce-$this->option" );
        set_theme_mod( $this->option, $_POST[ $this->option ] );
    }

    /**
     * Create the form elements.
     *
     * @wp-hook admin_footer-appearance_page_custom-background
     * @return void
     */
    public function form()
    {
        $nonce = wp_nonce_field(
            $this->option,
            "_t5nonce-$this->option",
            TRUE, // check referer
            FALSE // do not echo
            );
        $html = $nonce . $this->get_radio_fields();
        $this->print_script( $html );
    }

    /**
     * Create the jQuery function that inserts our form fields.
     *
     * @param  string $html Radio buttons
     * @return void
     */
    protected function print_script( $html )
    {
        $row = "'<tr><th>$this->table_header</th><td>$html</td></tr>'";
        ?>
<script>jQuery( function <?php echo $this->option; ?>($) {
    $('.form-table:last').append(<?php echo $row; ?>);
});</script>
<?php
    }

    /**
     * Helper for form(). Create radio input fields
     *
     * @return string
     */
    protected function get_radio_fields()
    {
        $value  = get_theme_mod( $this->option, 'padding-box' );
        $radios = array ( 'padding-box', 'border-box', 'content-box' );
        $html   = "<p>";

        foreach ( $radios as $radio )
        {
            $html .= sprintf(
                ' <input type="radio" name="%1$s" value="%2$s" id="%3$s"%4$s>'
                . ' <label for="%3$s">%2$s</label> ',
                $this->option,
                $radio,
                "$this->option-$radio",
                // returns ' as value delimiters and has to be escaped
                addslashes( checked( $value, $radio, FALSE ) )
            );
        }

        return "$html</p>";
    }
}

La respuesta proporcionada por @toscho me pareció muy útil, pero como tenía más de una opción para agregar, modifiqué un poco el código para que todo lo que tengo que hacer es crear una Clase extendida simple con algunas opciones.

También me pareció inconveniente tener las opciones simplemente agregadas al final de la lista, así que agregué un argumento de 'posición' que le permite seleccionar cualquiera de estas opciones:

• 'first' - Antes del primer ajuste (posición actual)
• 'last'- Después de la última configuración (Color de fondo actual)
• Integer position - El número de fila para insertar la configuración antes (debe ser un número entero)

Aquí está el código

add_action('load-appearance_page_custom-background', array('PS_Background_Setting_Random', 'get_instance'));
add_action('load-appearance_page_custom-background', array('PS_Background_Setting_Position_Y', 'get_instance'));
add_action('load-appearance_page_custom-background', array('PS_Background_Setting_Size', 'get_instance'));

/**
 * Add a new 'Random Background' setting to the Customise Background admin page
 */
final class PS_Background_Setting_Random extends PS_Background_Setting{

    /**
     * The main instance
     *
     * @var object|null
     */
    protected static $instance = null;

    /**
     * Return an instance of this class
     *
     * @return object   An instance of this class
     */
    public static function get_instance(){

        NULL === self::$instance and self::$instance = new self;
        return self::$instance;

    }

    /**
     * Constructor
     */
    public function __construct(){

        $args = array(
            'mod'       => 'ps_background_random',
            'default'   => 'yes',
            'label'     => __('Random Background', 'djg_photo_show'),
            'position'  => 'first',
            'options'   => array(
                'yes'   => __('Yes', 'djg_photo_show'),
                'no'    => __('No', 'djg_photo_show')
            )
        );

        parent::__construct($args);

    }

}

/**
 * Add a new 'Background Position (Y)' setting to the Customise Background admin page
 */
final class PS_Background_Setting_Position_Y extends PS_Background_Setting{

    /**
     * The main instance
     *
     * @var object|null
     */
    protected static $instance = null;

    /**
     * Return an instance of this class
     *
     * @return object   An instance of this class
     */
    public static function get_instance(){

        NULL === self::$instance and self::$instance = new self;
        return self::$instance;

    }

    /**
     * Constructor
     */
    public function __construct(){

        $args = array(
            'mod'       => 'ps_background_position_y',
            'default'   => 'cover',
            'label'     => __('Position (Y)', 'djg_photo_show'),
            'position'  => 3,
            'options'   => array(
                'top'       => __('Top', 'djg_photo_show'),
                'center'    => __('Centre', 'djg_photo_show'),
                'bottom'    => __('Bottom', 'djg_photo_show')
            )
        );

        parent::__construct($args);

    }

}

/**
 * Add a new 'Background Size' setting to the Customise Background admin page
 */
final class PS_Background_Setting_Size extends PS_Background_Setting{

    /**
     * The main instance
     *
     * @var object|null
     */
    protected static $instance = null;

    /**
     * Return an instance of this class
     *
     * @return object   An instance of this class
     */
    public static function get_instance(){

        NULL === self::$instance and self::$instance = new self;
        return self::$instance;

    }

    /**
     * Constructor
     */
    public function __construct(){

        $args = array(
            'mod'       => 'ps_background_size',
            'default'   => 'cover',
            'label'     => __('Size', 'djg_photo_show'),
            'position'  => 6,
            'options'   => array(
                'auto'      => __('Auto', 'djg_photo_show'),
                'contain'   => __('Contain', 'djg_photo_show'),
                'cover'     => __('Cover', 'djg_photo_show')
            )
        );

        parent::__construct($args);

    }

}

/**
 * Add a new setting to the Customise Background admin page
 */
class PS_Background_Setting{

    /**
     * The name for the theme modification option
     *
     * @var string
     */
    private $mod = '';

    /**
     * The default value to return if $mod is not yet set
     *
     * @var mixed
     */
    private $default = false;

    /**
     * The label for the additional setting
     *
     * @var string
     */
    private $label = '';

    /**
     * The options to use for creating the fields for the additional setting
     *
     * @var array
     */
    private $options = array();

    /**
     * The nonce for the additional setting
     *
     * @var string
     */
    private $nonce;

    /**
     * The HTML fields for all of the options for the additional setting
     *
     * @var string
     */
    private $fields;

    /**
     * The position in which to insert the option
     *
     * @var string
     */
    private $position = 'last';

    /**
     * Constructor
     */
    public function __construct($args = array()){

        /** Map the args to this object */
        foreach($args as $key => $value) :
            $this->$key = $value;
        endforeach;

        /** Ensure that all of the required $args are valid */
        if(!$this->is_valid_args()) :
            return;
        endif;

        add_action('admin_footer-appearance_page_custom-background', array($this, 'output_additional_setting'));

        /** Check to see if there is an option to save */
        if(!empty($_POST[$this->mod])):

            /** Check the nonce is valid and save the updated setting */
            check_admin_referer($this->mod, "_ps_nonce-$this->mod");
            set_theme_mod($this->mod, $_POST[$this->mod]);

        endif;

    }

    /**
     * Ensure that all of the required $args are valid
     */
    private function is_valid_args(){

        return (empty($this->mod) || empty($this->label) || empty($this->options)) ? false : true;

    }

    /**
     * Output the additional custom fields to the custom backgrounds page
     */
    public function output_additional_setting(){

        $this->nonce = wp_nonce_field(  // Create a nonce for each settings so that it can be checked when the user saves
            $this->mod,             // The nonce $action
            "_ps_nonce-$this->mod", // The nonce $name
            true,                   // Also create a referer nonce
            false                   // Do not echo
        );
        $this->set_fields();    // Set up the fields for this setting
        $this->print_script();  // Print the jQuery that will insert the setting into the DOM

    }

    /**
     * Create the HTML fileds for all of the options required for the additional setting
     *
     * @return string   The fields for the additional setting
     */
    private function set_fields(){

        $saved_value = get_theme_mod($this->mod, $this->default);

        foreach($this->options as $value => $description) :

            $checked = ($value === $saved_value) ? 'checked="true"' : false;

            $fields[] = sprintf(
                '<label>'.
                '<input type="radio" id="%1$s" name="%2$s" value="%3$s" %4$s>'.
                '%5$s</label> ',
                "$this->mod-$key",  /** %1$s - The option ID */
                $this->mod,         /** %2$s - The option name */
                $value,             /** %3$s - The option vale */
                $checked,           /** %4$s - Whether or not the option should be checked */
                $description        /** %5$s - The option description */
            );

        endforeach;

        $this->fields = join('', $fields);

    }

    /**
     * Create the $row to insert in to the DOM and the jQuery function to carry out the insertion
     */
    private function print_script(){

        $row = sprintf(
            '<tr>'.
            '<th scope="row">%1$s</th>'.
            '<td><fieldset>'.
            '<legend class="screen-reader-text"><span>Background %1$s</span></legend>'.
            '%2$s%3$s</fieldset></td></tr>',            
            $this->label,   /** %1$s - The setting label */
            $this->nonce,   /** %2$s - The nonce field for this setting */
            $this->fields   /** %3$s - The setting fields */
        );

        if(!is_int($this->position)) :
            $this->position = (in_array($this->position, array('first', 'last'))) ? $this->position : 'last';
        endif;
?>
<script id="custom-background-mod-<?php echo $this->mod; ?>">jQuery(function <?php echo $this->mod; ?>($){

    /** Insert the '<?php echo $this->mod; ?>' setting option in to the Custom Background admin page */
    var row = '<?php echo $row; ?>';
    var rows = $('.form-table:last tr');
<?php if(is_int($this->position)) : ?>
    var position = parseInt('<?php echo $this->position; ?>');
    if(position < 0){
        rows.first().before(row);
    }
    else if(position > rows.length){
        rows.last().after(row);
    }
    else{
        rows.eq(position - 1).before(row);
    }
<?php elseif($this->position === 'first') : ?>
    $('.form-table:last tr:first').before(row);
<?php else : ?>
    $('.form-table:last tr:last').after(row);
<?php endif; ?>

});</script>
<?php
    }

}