Storefront- Elegance add subject field

Hi there,

I’m hoping someone on here has used the Storefront-Elegance theme before!

What I’m wanting to do is add the subject field to the backend for customer purchase receipt. I’m not sure which file to edit or how to get it to work. If anyone could point me in the right direction, I’d be really grateful! At the moment the fields are email, name and message body.