If the design is purely done with css then you won't have to give any file. Just let the designer modify your live site using firebug. You don't have to give him/her access to your admin. Just let him document the process so that he can provide a redesigned css. I have not yet tested this workflow but perhaps it will work since he can do it with a functional site. Also, he is not be restrained with codings or deal with php codes.
Again, it is just based on my ways of modifications and perhaps there might be some more efficient way to do it.
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