It keeps the changes if I upgrade the theme?

Hey pnacho!

No, it will be overwritten. Please use Child theme if you would like to make changes on theme files – http://kriesi.at/documentation/enfold/portfolio-item/create-a-child-theme/

Best regards,
Yigit

Hey!

The change or modification on the link you provided should be done on avia.js file not on function.php. If you want to modify the avia.js file on the child theme, add this on the child theme’s functions.php:

if(!is_admin()) add_action('wp_enqueue_scripts', 'avia_register_child_frontend_scripts', 100);
function avia_register_child_frontend_scripts()
{
	$child_theme_url = get_stylesheet_directory_uri();
	wp_dequeue_script('avia-default');

	//register js
	wp_register_script( 'avia-default-child', $child_theme_url.'/js/avia.js', array('jquery'), 1, true);
	wp_enqueue_script( 'avia-default-child' );
}

Create a js folder then insert the avia.js file.

Cheers!
Ismael

Hi!

It enqueues avia.js file from your child theme and does not use the one on parent theme. You do not need to make this change for all files

Cheers!
Yigit

Hi!

Exactly. For more information, please see – http://codex.wordpress.org/Child_Themes

Regards,
Yigit

Hey!

You can use Style.css file of your child theme to add your custom CSS code, you do not need custom.css file.

Regards,
Yigit