Mike, the developer of Cesium, contacted me about my earlier post about Cesium navigation. Mike said it was a small bug affecting how the app reads the preference for the theme. He found the bug shortly after the app was released, and has fixed it, but the fix hasn't gone out in an update yet. He has a large update due for release and it should be out next week.
He suggested if I change the theme, it will get past the glitch and the navigation will reappear. I did that, and look at the difference. The first image is the before, and the second is after switching theme and then switching back.
There is so much to like about Cesium. It's a really nice app, and it works the way my mind works. The only problem I have experienced with the app has been this very minor visual glitch that is easily worked around. To make the experience even better, the developer is actively responsive and seems like a really nice person.