In that case, ignore those messages in the terminal - they're probably not important. The crux of the matter is how you change the relevant .css file and then get it to stick. Which theme are you currently using - Mint-X? If so, you need to be changing the css of *that particular theme* (try Googling "changing cinnamon theme css" and see what you come up with).

Thank you. I think I see the problem I created for myself. I was not going into the cinnamon.css file within the particular overall theme I'm using - which is Linux Mint. I'll mess with that this evening and see how it goes.

Nope. No go. Even when I access cinnamon.css through the theme folder (in my case Linux Mint) the changes don't stick and I get this return in terminal:

ken@ken-X200CA ~ $ gksudo gedit linux mint/cinnamon/cinnamon.css

** (gedit:4960): WARNING **: Set document metadata failed: Setting attribute metadata::gedit-spell-enabled not supported

** (gedit:4960): WARNING **: Set document metadata failed: Setting attribute metadata::gedit-encoding not supported

** (gedit:4960): WARNING **: Set document metadata failed: Setting attribute metadata::gedit-position not supported

** (gedit:4960): WARNING **: Set document metadata failed: Setting attribute metadata::gedit-position not supported

** (gedit:4960): WARNING **: Set document metadata failed: Setting attribute metadata::gedit-position not supported
ken@ken-X200CA ~ $

There must be a simple error that I'm making.

to sum your problem up in one sentence:
do not edit system files (at least not in this case).
(i am pretty sure that's why the mint update gave you trouble)

instead, copy the theme in question to the appropriate place in your $HOME and apply changes to it there.
usually it would be /usr/share/themes/sometheme to $HOME/.local/share/themes/sometheme etc.

these files can be then edited without superuser privileges.

@ondoho: And then.....

sorry but you lost me there.

It doesn't matter, really. I just wanted to change a few lines of java script in a .css file and it's turning into something like I want to re-program the Cassini probe or something.

I should have known when it started like this: the text editor installed by default with Mint 18.3 is gedit. Someone pipes up that actually the proper editor to use with Mint is xed. Right then I should have just walked away.

That is not what I said. I said that "The default editor in Mint (Cinnamon, MATE, Xfce) is xed." If you install Linux Mint 18.3 and, without changing anything else, you choose Text Editor from the system menu then xed is the editor used. That doesn't make it the "proper" editor. I just thought that you would be interested in this information. I didn't expect to be accused of "piping up".