In your command s/\/fonts/../fonts/ is being taken as the parameter to the -i option (the suffix to use for the backup file), and the filename argument is being treated as the editing commands.
You need to specify to disable the backup file creation:
sed -i '' ...
In your example:
sed -i '' 's/\/fonts/../fonts/' /Users/sergeybasharov/WebstormProjects/snap/compiled/Content/stylesheets/style.css
Computers are dumb, they don’t figure things out by context, so they can’t tell that something beginning with s/ is obviously an editing command, not a suffix.