Generally, I would recommend creating your own buttons via normal JS/HTML/CSS (or using whatever toolkit you want). The safest approach would be build your UI outside of the cesium container, and then use CSS to position your UI elements over top of the stock UI in whatever desired location. Alternatively, you can dig around and find the toolbar div and appendChild directly to that element. I’d suggest examining the source code to see what the different DOM elements are used for, and to make sure your elements won’t get clobbered, since it’s generally not documented.