I did find an answer to my question. One can change the style by using a private style sheet and adding the Local definition for style "Inactive" from the Core style sheet. (You have to dig down through the Default style sheet to get the Core style sheet.) THe following changes it to Black.
Cell[StyleData["Inactive"],
FontColor->RGBColor[0, 0, 0]]
But still I wanted a darker style in Help pages I'm writing in Workbench. I do think the default "Inactive" style should be a darker color.