In presentation mode (F5) I see my document surrounded by black color (sorry, I unknow the correct term. Sides? Canvas? Margins?).
Is there a way to change that color? Because I failed to find one.
Please notice I’m not talking about BackgroundColor (the color of the page, let’s say) nor MainWindowBackground.
Thanks in advance 
No
same as for images such as comicbooks that canvas is black (unless you press w for whiteout)
Easier to use F11 with gradients see
Not many users may be aware that if you “Save as” a folder of images, which also means a comic book folder archive (e.g. comic.cbr), to a PDF. Then fairly quickly that PDF could be “presented” with the above gradient background settings in Fullscreen mode. However that ability is not currently available if the folder/archive includes .webp images.
Thank you so much for your prompt answer and pointers 
Regards!