Automatic switch of paper and ink colour between dark and light modes
In dark mode, switch automatically to dark paper and white ink, and revert back automatically to light paper and black ink when in light mode.
I think it's fine to leave all the other colors unchanged (besides the highlighters, they are visible only until you lift the pen, so maybe it's more an issue of how to render them on dark paper). If you export without background (e.g. if you send your document to someone who prefers a white background, or if you want to print it), white ink is the only color* that won't be visible anymore. Therefore it should be converted automatically.
* as long as you don't add the colors of the yellow and dark paper to the standard color palette.
I think it would make more sense to leave the paper in the color that it is and also not switching ink colors. That way, your note would look the same in all scenarios. Colors like dark green aren't really visible on black. I think it's enough to make the interface and menu (and the margins next to the page) darker, but not the page itself. If you want black paper also, you can already chosse a "dark" color option for the paper in your note settings.
Any idea of how that would work with coloured writing, i.e. green not black?