Here is my first implementation of pokkis's advice, using
davidforr's "random screensaver" "hack.
P.S. I came across another way of creating your own screensavers by frostschutz at
https://www.mobileread.com/forums/sh...82&postcount=1 . When I look at the last 2 attached images (as thumbnails), I like it a lot, because the image is black on white while sleeping (just like Kobo's default) and white on black while powered off (again, just like Kobo's default). But I wonder if that 2015 tip is still applicable in 2019, or if there's a better way to do what frostschutz has implemented.