CUSTOMIZING CONTACT FORM

Hi!
I am wondering to customize contact form on listing page. I am using Classified Theme. Can you tell me which file do I need to customize? and Hints would be appreciated !
The reason for I want to update is
– When visitor fill out and try to contact listing author,
Website admin email address will use on email
when listing author reply, email will send to website admin , Not to whoever fill out that contact form.

That’s ridiculous.
On next version please fix this.
[ NOTE: don’t want to use third parties plugin ]