Hi there,
the file structure has to be like Amit desribed it and Karen’s CSS does the trick, thanks!
child theme folder > assets > css > gutenberg > gutenberg-editor.min.css (this doesn’t have to be copied, it just has to have the same name).
I suppose the OceanWP Devs made this styling in order to make the back end editor look more similar to the front end text in the theme (in the basic setting without changes to the typography settings).
But I agree – it makes the fonts in the Gutenberg editor quite illegible and editing unnecessarily cumbersome; please at least increase the contrast of the font there.
Best