theme is never changed after pressing the button

https://developer.ubuntu.com/api/apps/qml/sdk-15.04.1/Ubuntu.Components.ThemeSettings/

on the page, the code has a little problem. The default theme is:

Ubuntu.Components.Themes.Ambiance

There is no chance to change the them at all according to the code. I have modified the code a little bit:

        Button {
            text: theme.name == "Ubuntu.Components.Themes.Ambiance" ?
                      "SuruDark" : "Ambiance"
            onClicked: {

                theme.name = (text == "Ambiance" ?
                       "Ubuntu.Components.Themes.Ambiance" :
                       "Ubuntu.Components.Themes.SuruDark" )
            }
        }

With the code, it works fine.