Ubuntu has released the Ubuntu User Interface Toolkit, which is listed in the References section. It tells you how to import needed parts into your app when you are writing it. It tells you what various classes do, how they work, and notes where they may still be in development so you can watch for changes.
Included topics are:
General Topics, such as layout and resolution independence so that apps look good regardless of the type of device they are used on.
Basic QML Types, such as buttons, checkboxes, and labels. ...