styles

//controls/Action.decl
Action : Object {
    property bool checkable;
    property bool checked;
    property bool enabled;
}

//Global
ButtonStyle : Item {
    id : defaultButtonStyle;

    Rectangle {
        anchors.fill: defaultButtonStyle.parent;

        color: defaultButtonStyle.parent.Button.checked ? "#000000" : "#FFFFFF";
    }
}

//controls/Button.decl
Button : Item {
    id: root;

    property alias checkable: action.checkable;
    property alias checked: action.checked;
    property ButtonStylePtr style;

    Action {
        id: action;

        enabled: root.enabled;
    }

    onSelectPressed: {
        if (enabled) {
            if (checkable)
                checked = !checked;
        }
    }

    onCompleted: {
        if (!style)
            self->style = Globals()->defaultButtonStyle;
    }
}

//myapp/styles/ButtonStyles.decl
CustomButtonStyle : ButtonStyle {
}

//myapp/MyApp.decl

CustomButtonStyle {
    id: customButtonStyle;
}

Button {
    id: myAppButton;

    style: customButtonStyle;
}