I can only guess, but most likely this has to do with your server settings and file permissions.
When you install the template in Joomla, the archive is uploaded and extracted on your server. The files are created as your default web user (depends on your server settings).
If that user is not the same as the ftp user, the ftp user can not alter them…
So maybe try setting the whole template directory (with contents) to something like 777, or login on SSH and change the user/group permissions of the template directory, so the FTP user has write permissions.
Another option would be to use the file editor in CPanel / confixx, or whatever administration panel your hoster uses.
Hope that helps.
