Could do, but why?
In the past if you wanted the ACE editor to display differently from default you used userchrome.css to tweak it. Just wondering why that should be any different with the CodeMirror editor?
I have only had a quick look at this but it seems that just these few lines of css in userchrome.css will set CodeMirror to the plainest, non-enhanced, text-editor-only style display settings.
div.CodeMirror-lines * {
font-size: 13px !important;
color: black !important;
font-weight: normal !important;
text-decoration: none !important;
}
Actually userchrome.css will let you change pretty much everything in CodeMirror from the default.