How to enable Custom Fields that are disabled by theme?

Some one already asked that here, but that doesn’t solve my problem.

My theme disabling the custom fields. How I can enable it manually? I want to know that which code WordPress use to show Custom fields widget? Please help.

Update: Actually this is educational theme that create some new Posting options which are “Course”, “Lesson” & “Testimonial”. I want to bring custom fields there.

Newly created posting methods
Now custom fields option

Fuctions.php:

    <?php
//Error reporting
error_reporting(E_ERROR | E_WARNING | E_PARSE | E_COMPILE_ERROR);

//Define constants
define('SITE_URL', home_url().'/');
define('AJAX_URL', admin_url('admin-ajax.php'));
define('THEME_PATH', get_template_directory().'/');
define('THEME_URI', get_template_directory_uri().'/');
define('THEME_CSS_URI', get_stylesheet_directory_uri().'/');
define('THEMEX_PATH', THEME_PATH.'framework/');
define('THEMEX_URI', THEME_URI.'framework/');

//Set content width
$content_width=1140;

//Load language files
load_theme_textdomain('academy', THEME_PATH.'languages');

//Include theme functions
include(THEMEX_PATH.'functions.php');

//Include theme configuration file
include(THEMEX_PATH.'config.php');

//Include core class
include(THEMEX_PATH.'classes/themex.core.php');

//Init theme
$theme=new ThemexCore($config);

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Create a Child Theme, Plugin or Must Use plugin, and use add_post_type_support:

<?php
/* Plugin Name: Add CF to CPTs */

add_action( 'plugins_loaded', 'add_cpt_support_wpse_116891' );

function add_cpt_support_wpse_116891(){
    # See /wp-admin/edit.php?post_type=SLUG
    add_post_type_support( 'SLUG-POST-TYPE-1', array( 'custom-fields' ) );
    add_post_type_support( 'SLUG-POST-TYPE-2', array( 'custom-fields' ) );
    add_post_type_support( 'SLUG-POST-TYPE-3', array( 'custom-fields' ) );
}