I still don't see it
Under preferences-->look and feel, I see 2 choices under "user interface style". One is "System default" and the other is "calibre style". I switch back and forth between these two, allow it to restart, but do not see a difference between the two.
Neither style looks like the sample screenshots. For example, the sample screenshot shows every other book listed with a grey background or a white background. My version of the software just has all books listed with a white background.
Also the book viewer has a night mode. I cannot figure out how to get that night mode in my book viewer.