Try this: start Rhino, open Rhino preferences, display modes, go to ‘shaded’ (or any other display mode that hasn’t been altered) and set surface edge color reduction to 70 % (for instance), exit Rhino preferences. Make two boxes (or whatever objects) side by side - or overlapping (so you could boolean them in to one object). Click on the common edge between the boxes and make sure that the object selection box pops up. Notice that the highlight color is the normal pink. Save the file.
Exit rhino, start Rhino and open the previous file. Try the above selection-highlight-test with the overlapping objects. The highlight color isn’t pink anymore, but almost black, which makes it impossible to see what you are supposed to select. It seems that the edge reduction color also ‘reduces’ the pink to a 70 % black - which it shouldn’t.