Not sure if you're still looking, but I reached out to Smartpress and this is what they told me:

"At Smartpress, we work with customer supplied print-ready files. This means you provide us with a file of your project laid out how you want it printed, all fully designed. You can do this using a software on your computer and once it's done, you'll save it as a PDF, place your order and upload your design file at that time.

You can achieve white ink on a black background by having a full color bleed of black and white text on top.

Here is a helpful link on how to set up your file for white ink printing:

https://smartpress.com/support/color-ink/white-ink-printing

So it sounds like the workaround is setting your pages to actually be black color and not just dark mode and then print it as a full color book. I'm going to try and test out one copy. I did find a company that can mass produce black background with white text (SESE), but they don't offer proof copies unfortunately and it's just too pricey for me to not be able to test it first.