enable HTML in emails in Appointments+

I am trying to enable HTML in emails in Appointments+

I have found the following in your FAQ
-----------------------------------
Appointments+ uses wp_mail function and HTML is disabled as default. To enable HTML in emails, add these codes inside functions.php:

function app_modify_headers( $headers ) {
return str_replace( 'text/plain', 'text/html', $headers );
}
add_filter( 'app_message_headers', 'app_modify_headers' );
-----------------------------------

I am using the Avada theme. So I went to the folder wp-content\themes\Avada and I added the code to the functions.php file in this folder.
However, I still get emails in plain text.

Any ideas?