Friday13th, all your answers have been answered in this thread, and in this one:
https://www.mobileread.com/forums/sho...d.php?t=170312
Please spend some time reading through the threads [moderation edit] and you'll find many of your current answers answered.
The JBL is a great device! My only gripe about it is it's weight, and white shiny border, rather than the screen background.
The background is more like an old palmtop or hand calculator - grey, but the blacks are really a lot blacker; giving you a much better contrast ratio than a calculator.
It's pretty readable, even in low light conditions.
The only thing that's hard to read is where colors are mixed like light green, with light blue, and yellow and orange, it's rather difficult to differentiate between all of them on the Jetbook)