I changed the first two files
viewer.json -> viewer.json.save
viewer.py -> viewer.py.save
since you don't show changing the third one, I didn't change it.
Opened Calibre, opened a mobi book in the viewer and it looked okay with a white background, switched to paged view, still looked okay with a single page, changed to 3 columns and got this
fullscreen
Next I'll change all three files and try again.