"Customizing Qt Quick Controls 2" documentation lacks information

On https://doc.qt.io/qt-5/qtquickcontrols2-customize.html there is the part where "We then copy the existing default Button style, and add the code for a drop shadow" but it does not say where to copy the file (and from where would be helpful as well) and how to make it work within the project.

Furthermore if I then copied the file and inserted the code snipped shown, I get an error, that "DropShadow" is not known. Maybe there is a version mismatch? I have 5.7 installed.