It's a good idea to have a "Test Shop" installed somewhere, so that you can try things out first, before you "wreak potential havoc" on your client's store :D .
This is easy to set up... Just create a separate database and do a ZC install in a SUB-directory of your existing host.
If your host is to miserly to allow you more than one database, you can do an install using the database "prefix" function, but that is not advised if your second install is for the purpose of testing and training.
Rather create a completely separate database...
Also, when you EDIT a file, ALWAYS first make a back-up (copy) of the original. It is surprising how many people go "bull in a china shop" with the code files, and then when their webshop falls over, can't remember what or how they changed things. If you have a backup of an original file (including the css files), then you simply invoke the original again, while you pick apart your edited version to see where you went wrong.
When I edit a "php" file, I first copy it to my hard drive, then edit it and save it on my hard drive.
I then upload it back to the server into the applicable "over-ride" folder.
If a copy of that file already exists in the over-ride folder, I re-name that file by giving it a different file extension, eg:
tpl_main_page.php
would be re-named:
tpl_main_page.OLD
... only then do I FTP the edited version of tpl_main_page.php .