I’ve noticed in Rhino 8 that curves/lines display as faded (ie less bright) in model view compared to 7.
It’s most noticeable with the indigo walls in the screenshots below (both wireframe views of 2D linework). Note that dimensions, hatches and text are equally bright between the two versions.
Rhino 7:
Rhino 8:
I find that in layout view too, the lines are more difficult to see. The only setting I could find that might affect this is PrintDisplay, but it is turned off.