Autoresponder subject changed in functions.php

Gday Kriesi

I understand that the contact form autoresponder subject can be modified in the contact.php file

However, I would prefer not to modify files directly, instead working with a child theme.

Could someone please help me regarding code to allow this modification to take place by using the functions.php file of my child theme, eg. with action hooks/filters?

In addition, it just occurred to me, if I use multiple forms, does this autoresponder text get applied to every form autoresponse?

cheers

Darryl