JCE is a great editor, but sometimes hard to configure.
My eyesight for PC screens has been getting worse. I'd never worked out how to get the editor to use the same fonts set in the template. but with around 6pt fonts that I just couldn't see anymore, it was time to work it out!
There is a setting in "global configuration" about css files that was already set to /templates/$template/css/editor.css
It appears that jce doesn't use that setting but there's a different one:
select editor profiles->default->editor paramters->typography
"custom css files" should also be set to /templates/$template/css/editor.css
And that was it!