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I can not imagine any circumstance in which file system access would be necessary to create a style. Giving them admin access to a private(password protected) install would probably be best. If they say they need it for images then give them a blank directory.
xF License Agreement said:
You may create a single additional test installation for the purpose of testing the Software. Any test installation of this kind must be password protected, and access to it must be limited to You and Your website staff.