If you are curious, I recently open sourced the app. It's called Keynote Extractor. Here's the repo.

The kind of work is to study how dark mode is implemented and to change assets based on the dark mode API. There's a few icons in there and basically it's a single view app.

I don't have Mojave yet so I don't know how much is automatic but I assume most of the app will look broken on Mojave in dark mode.