Add default Prose design settings values.

functions.php

<?php

add_filter( 'prose_settings_defaults', 'custom_prose_settings_defaults' );
/**
 * Add default Prose design settings values.
 *
 * These values are used when options are reset, or an invalid value is submitted.
 *
 * @author Gary Jones
 * @link   http://code.garyjones.co.uk/set-prose-defaults/
 *
 * @see prose_settings_defaults() Contains the prose_settings_defaults filter hook.
 *
 * @param array $defaults Array of default values with the key as the mapping name.
 *
 * @return string
 */
function custom_prose_settings_defaults( $defaults ) {

    $defaults['code_background_color'] = '#FFE09E';
    $defaults['code_font_family']      = "'Courier New', Courier, monospace";
    $defaults['code_font_color']       = '#222222';
    $defaults['code_font_size']        = '12';
    $defaults['code_border']           = '2';
    $defaults['code_border_color']     = '#F9AB1F';
    $defaults['code_border_style']     = 'dotted';

    return $defaults;

}