Customise Wordpress Theme

I've purchased a Wordpress Theme that has a basic structure and facilities which are great.

I need somebody who has a great understanding of PHP, HTML and CSS to rework into the template, and modify it to accommodate a couple of design aspects I would like to include.

Really there should be no need to touch any of the PHP and hopefully you can alter the design without touching the theme's functions and facilities but just looking through and altering the HTML and CSS.

Is this even possible? Give me a reply with your thoughts on this (rather than a generic response).

I can provide a PSD showing exactly how I want to alter the design.

The website design is currently just a unfixed box in the middle. Here are examples of what I want to add:
- A new header above and outside the box, stretched.
- A new footer above and outside the box, stretched.