Try:

Print > Uncheck the option "Rewrap contents to fit page" > PDF > Save to desired location

This seems to retain the original formatting. As far as background goes, I doubt it is an option to natively export a dark mode themed PDF as printing such a page would be a massive waste of ink. You would have to screenshot your note and print the screenshot to save the dark formatting. 

Standard screenshot:
CMD + Shift + 3