I found the current color for constraint relation line is hard to see a little bit.
I've tried to change the "wire"color in the Theme setting panel, but it seems not work for the relation lines.
Thanks for your guide.
I see your point. It puzzled me a lot, too.
Currently I found that the only way to change the relationship lines for constraints are nothing but the "Grid", not the "Wire".
But, This is really not good for distinguishing the constraint lines from the background, including grid, by doing this. It seems that they are mixed together.
I also hope it may draw dev's attention to improve this. As I said, it's really not good to tweak two different thing by one single option.