Themeing the Backend Help

I have been asked to change the appearance of the backend pages regarding creating/editing new posts.

This affects the following files.. (as far as I can tell)

post-new.php
post.php
edit.php

I have been able to successfully “theme” or style these pages to reflect the front end design. Now here is my problem:

Whenever a user views the “post-new.php” page or “edit.php”, they are able to do whatever they need (i.e. make a new post, or edit an existing post). Upon clicking “update post” from the edit.php page, or upon clicking “submit” from the post-new.php page they are redirected through post.php, which will typically show them their post and what they have just done or something similar. Instead however they are getting this error:

Warning: Cannot modify header information - headers already sent by (output started at /public_html/toolbox/wp-admin/post.php:8) in /public_html/toolbox/wp-includes/pluggable.php on line 897

(notwithstanding this error, their new post or edited post still takes effect and actually works)

Typically I understand that this problem is usually a result of white space or something else being placed before the opening php tag.

On the post.php, I expressed that this was a page that needed to be “themed” to reflect the front end. So naturally there is html before the <?php.

What do I need to do to get around this?

So ultimately..
How can I theme the post-new.php and edit.php page to reflect the frontend?

Thanks!