QmlDesigner: Register type for QtQuickDesignerTheme

This was for some reason working before, but should not have worked.
Qt 6.7.2 does require this.

Change-Id: I599e5d8ee3535eb633a08bb0d5605e70cbf42097
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/componentcore/theme.cpp

@@ -35,6 +35,7 @@ Theme::Theme(Utils::Theme *originTheme, QObject *parent)
               .toString();
 
     QQmlEngine *engine = new QQmlEngine(this);
+    setupTheme(engine);
     QQmlComponent component(engine, QUrl::fromLocalFile(constantsPath));
 
     if (component.status() == QQmlComponent::Ready) {