As first could you please tell me where exactly is this CSS file stored? Is it stored in the file system or are you using the Site Manager -> Development -> CSS Stylesheets?
Because sync of physical files is not supported yet: What can be synchronized
The only place would be to place the files into the content tree or as a document attachments or as a media library files (however, changes made directly to the physical file are not logged as a content staging task).
Maybe, you should use some other tool which keeps the physical folders in sync - something like DropBox.