There is new problem with the highlight color of gumball widget - the highlight color is always gray and it is impossible to change it. The essence of this new problem is that Rhino.Options.ChooseOneObject.HighlightColor variable does not control the highlight color of the gumball widget. If the background color is gray, the highlighted part of the gumball is invisible.
The Gumball highlight appears to follow the “Feedback” color here…
By default this color is full black (I think).
You are right. I was confused because this color changed recently, and I could not find any info in Rhino documentation. Rhino documentation is bad. Grasshopper documentation is worse.