I hadn't realized that the app was written in Python. I have some experience working with Python myself, so I went ahead and added the keybinding for previous/next page on keypad left and right in fit-width mode, and on keypad up and down when the screen is rotated. I also added a binding for the center button to the "smart scrolling" feature which is normally bound to the space bar.

I'll take a look at adding a customization dialog in the next day or two.