Referencing 2 older threads:
2016: Crosshair color & size - update? (https://forum.freecadweb.org/viewtopic.php?t=14597)
2013: Setup for Crosshair (http://forum.freecadweb.org/viewtopic.php?t=3722)
This would be a nice customization option.
Usecase: if the user has setup their background to be a lighter neutral color it's difficult to see the crosshair
Case in point:
Workaround: obviously to change the background color (but this is not an ideal solution hence the Feature Request)
Edit: issue #4115