You can currently change the background color of the app (not the PDF) in the configuration file. We don't currently have True Dark Mode but it shouldn't be too hard to implement.
I tend to use a custom PDF darkmode in Firefox for this very reason. I can't find a decent PDF viewer that supports it. (Linux)
I've been using Acrobat on a FireHD tablet with a dark mode plugin. It works quite well. I can't remember the last time I needed to inhibit the plugin to read something.