My host, who will no longer provide WP support, placed some Zip Files into my theme directory since their upload interface was receiving error notices.
I went in to edit code and at the bottom of the code window in Theme Editor is this:
If this file were writable you could edit it.
A former host/consultant told me to login via FTP and CHMOD the files in question to 777. That will let WP write to them. Your FTP application should have an option for file permissions or CHMOD.
I have access to the files via my host's account manager, not sure if this helps me.
Any good advise or help is appreciated.