It seems I wasn't clear about the fundamental problem with the color of the Chromecast icon. I think that previously, it was always white. Sometimes just an outline and sometimes filled in, but always white. White is good because the background is never white. With the recent changes, the light theme had it colored black. This is bad because some of the title bars are dark gray. Black on dark gray is not visible! Also, all the other icons in the title bar are always white.
Always white seems to be the right answer.