can anyone tell me how to change the UI user interface of MIT App Inventor?
not finding a css file
You should check the Ode.java file.
It will lead you to various UI related files such TopPanel.java , TopToolbar.java , StatusPanel.java etc.
It is not easy because ui is made with many files you can look at appengine folder for ui of app inventor
You should check
client/explore/youngandroid/projectlist.java file
You can use ya.css file for change user interface