I have several wp jobs lined up in which i will create a custom layout in photoshop based off an existing themeforest theme. I would supply you the base theme (installed on wp with dummy content ready to roll) , the psd of the new design, and all the login info.
I design the psds to be pretty close to the initial themes, all of the edits are essentially cosmetic (like move a div, change background image, css styling)
heres an example of a before and after to help you gauge the type of css work to expect:
initial base theme demo : http://foxhoundbandthemes.com/demo/
after applying my custom skin : http://chevywds.com/
if you are interested or have questions about it or the price let me know, thanks!