The appdev theme comes with 4 colors only and does not have the option of custom color unlike the later themes of ours. Changing to custom color is not too difficult either and you did the right thing by editing the default.css file.
Once you upload the file to the child theme folder, you will need to load the stylesheet using the code in the child theme functions.php file –
wp_register_style('style-custom-skin-css', get_stylesheet_directory_uri(). '/default.css', array('style-skin-css'), false, 'all');
wp_enqueue_style('style-custom-skin-css');
assuming you have the default.css placed in the root directory of the child theme. Pls give a try and let me know if you still have trouble. Do post a temp login in case the changes do not take effect.