1) In the meantime, make a new directory solely for the CSS file(s).
2) Make a new FTP account for the freelancer, giving him/her write permission (777) ONLY for the the CSS directory.
After the freelancer finishes their work, simply delete their FTP account, move your CSS file(s) back to wherever you originally had them.
Alternately, if it isn't only CSS files in question, but rather some kind of template like wordpress, just grant permission (777) only for that directory. However, if it is a template directory, before granting the freelancer access, I would first save a directory listing of the directory's contents and sub-contents, and then do so again once the freelancer finishes. Then I would compare the two to make sure no shells or otherwise malicious files were added.