The style works by scanning a directory of images and applying those images to the controls. For example, when a Button is created, the style will look for button-background.9.png (or button-background.png for non-resizable buttons). As an example of what that image could look like, here's button-background.9.png from the default assets:
When the user presses the button, the style then looks for button-background-pressed.9.png (or button-background-pressed.png). In the case of the default assets, that would look like this:
The .9.png extension represents 9-Patch images. Using this format, we're able to encode information about how the control should look when it is resized into the image itself. Combine this with the pre-defined file naming convention and you've got a style that can be customized without ever touching QML, giving designers complete control over what they do best, and allowing developers to focus on writing applications, not styles.
To provide a starting point for creating your own style, we've created a complete asset template in Sketch: