You are right...
you can replace in theme.php
to test the core contact menu. But it works very weirdly for me. Correct behaviour is redirected to contact.php, but there is the subject field and it's not saving its value after resending. (tested with bootstrap3 and it works).
So this is not so easy as I thought.
Oh, I know now - that theme uses special contact.php, not core...
Nevermind, it's complicated, I need more time. You will need to wait. It's too late and I can't tomorrow.