@CerpherJoe
I found out what I was doing wrong.
Simply, a 'style' can be looked at as applying to a basic set of controls and not as it's own set of controls.
Once configured correctly (below) one simply drags the basic controls to the canvas and they retain their 'un-stylized' look. Once you build and run the program, the resulting window will show the 'stylized' QML type. In my case, a 'slider'.
To configure a Quick Controls style to work:
Add QT += quickcontrols2 to your project .pro file (thanks @sierdzio !)
Edit your main.cpp file to turn the style on (in my case 'Material'):
#include <QQuickStyle>
QQuickStyle::setStyle("Material"); (in the main section)
Use the Design mode Imports window to add the following to the main.qml file:
import QtQuick.Controls 2.3
import QtQuick.Controls.Material 2.0
Add controls to the canvas as desired. Build. You will see the controls now in your chosen style.