[jackm8]

Well, there are a few solutions. It's how I got to like them. Dark mode can do black borders at times in comic mode. It's pretty buggy, and not always reliable. Then there's making them yourself beforehand.

— Extract pdf pages as images.
— Add black borders to images. (I use batch convert tool in Faststone using these settings, modifying resolution to target device)
— Convert images back to pdf.