Thanks! That is a somewhat related issue, but even though the background styling works for me as well (iframe.noteTextViewer{ background-color: #b3ecb8; }), it applies only to the iframe itself, not its contents. I.e., I could not change the font settings that way (maybe it is possible but I don't know how?)...
I can manually copy the HTML and paste it into a Markdown note, and so far it seems to work just fine, but I could not see a simple way to automatically / programmatically change the markup from HTML to Markdown of several notes.
Which brings up the question (or maybe prompts another feature request): why can't we import Evernote notes as HTML into notes using Markdown markup, but without converting the HTML to Markdown? It seems this approach could potentially solve the issue...