You missed to follow my suggestion in post #17
Download the file from the server to your hdd
Edit the file on your pc (good to have a text editor like notepad++, you can google it)
Upload the file - before uploading you could rename the file on the server, so if the edited file creates problem you can roll back to the working file by just renaming it to its original name, i.e. defaults.css > defaults_old.css; if newly uploaded file works fine, you can delete the old one.

It would be good to have the same structure of Zen Cart on your local pc.
Now, before doing anything else do as Celtic too suggested:
download a copy of your files to your hdd (drag and drop from right to left pane)
If not all the files, at least the includes folder.

You may want to download a fresh package of Zen Cart, so that you can recover some of the original files.
Of course that does not include any different template, or mod you may have installed.