Though my eyes aren’t in their best condition anymore, I have no problem at all with the outlining of the thumbnails. I have set the Appearance to Dark-mode and the Viewer background to 0.20. This gives me plenty of contrast and a crisp blue line around the thumbnails. When I change the Appearance and viewer background to pure white, it gets a bit more difficult to see where you are in the thumbnails (But I don’t like that setting at all anyway). However when setting the Appearance to Dark-mode and the viewer background to pure white to 1 It is still very convenient to follow were you are with the thumbnails
Also moving forward and backward through the thumbnails, using the arrow keys, serves me very well. If I want to keep an overview I use to swipe through the thumbnails in stead of using the arrow keys, or in practice use both (arrow and swiping).
In N-up mode it works very convenient as it is now.
So as the wildrover writes. “It is not easy to please everyone”. Playing a bit with the settings did it for me. I have and use different apps, and they all have their specifics when it comes to the UI-view and sizes of text for instance.
Nitro serves me very well in this regard. (ha, and in lot of other things too -;)
Ben