outside of a stack like kirigami, you would have to do all the code to make theming work yourself. It's not a feature of qml, by design. It is meant to design identical ui and appearance on all plaforms, to the application. You can define what a "button" is, how it behaves, in minute detail. But to then design one that can also be modified by existing themes is a big project by itself. To do it for all the kinds of ui elements you use is probably far more work than the app itself.

qt qml does include standardish controls, like material design, windows, etc, but the problem there is that the app will use the same control set on every plaform. Imagine a mobile app with a windows-like ui ;). And they don't color or change based on platform theming at all. Or you could write multiple apps, each using a control set for each target platform. You can also define colors thru variables and then manipulate them yourself. I wish linphone had done that much.