You are correct. As a new feature, the UI Kit is being frequently updated to make it more customizable. The underlying code, however, is not publicly available at this time.
I understand how this may present some issues with customization, but in the long run we believe it will lead to a more stable experience for our customers that are using our UI Kit. We will be adding more customization soon, but in the meantime you always have the option to use our core Chat SDK if you would like a more customized UI beyond what is possible with the UI KIt.
With the development being done on this feature it is likely this comment could quickly become outdated. For future users that come across this post please check our documentation here to see what is currently possible using the UI Kit.