Theme Editor, making file editable.

Hi! I hope somebody can help me out!

I have downloaded the “girl” theme to my blog and would like to edit it using the Theme Editor but I keep getting the “You need to make this file writable before you can save your changes. See the Codex for more information.” message. I have made the “theme” files writeable for owner and group on my ftp client, is there anything else I need to do to make it work? I use the FileZilla FTP client.

Thanks!