The function on the app is changing color themes only. But now, in ver.1.3.4, users can now access and edit material images and source code directly, so I think they can customize all UI parts if they want to.
For example, how to rewrite a character image is as follows. :)
https://1oogames.itch.io/lovely-composer/devlog/616009/134-released-now-runs-pyt...