I'm working on a test site, on which I did a fresh install with the twenty thirteen theme. I have not installed any plug-ins.
The first thing I did was try to install the paid theme. (Avada) I kept running into a file upload limit, so I followed option 1 on this page, which is to add three lines of code to the functions.php file.
I did this through Appearance > Editor in the back end. But as soon as I clicked to make the change active, I got a 500 Internal Server Error message. Tried to get to a few other pages, kept getting that error. Logged out; now I can't even log back in. I get the log-in screen, but when I click log in, I get the same error page. So any solution that involves the back end won't work.
I went in through ftp (via Filezilla) and looked for the functions.php file in twenty thirteen, but I'm not finding it.
I don't currently have access to the host -- and would rather not bother the client with that if I can find another way to fix it.