It looks like it’s saying there’s a stray = sign on the first line of your custom css file. It should be…
/* Put any custom styles here. They will override the default styles. */
But it sounds like it’s seeing
=/* Put any custom styles here. They will override the default styles. */
If you open that file in an editor, do you see the stray equals sign? You can edit it on the server shell using a text editor like nano:
Worst case, you can just copy any existing styles somewhere safe (a notepad editor or whatever on your PC) and delete everything in the file. That should let you get the game back up and running again, and you can use the web portal to paste things back in bit by bit.