If you set the style for a Qt Quick Control, say a button for example, this particular style can only be seen in Qt Quick Designer if it is inside another component. If you are viewing the component that directly contains the control it doesn't show correctly, it just shows a regular control without any styles associated to it.

Example:

----- MyComponent.qml ------
import QtQuick 2.0
import QtQuick.Controls 1.1
import QtQuick.Controls.Styles 1.1

Button {
x: 10
y: 10
width: 100
height: 100
style: ButtonStyle {
background: Image

}
}

----- Main.qml ------
import QtQuick 2.0

MyComponent {
x: 0
y: 0
}

----- end of example ------

If you edit Main.qml in Qt Quick Designer you can view the Button with its style. However, if you edit MyComponent.qml you see a regular button without any style.