thank you Paul Andrew!
5) SharePoint themes have been detected in this project. The handling of themes has been changed in SharePoint 14. SharePoint 12 themes can be activated in backwards compatibility mode by setting SPWeb.UIVersion = 3 and subsequently calling SPWeb.Update().
Due to changes in the handling of themes in SharePoint 14, it is necessary to use the backwards compatibility mode. Otherwise, these themes will successfully be deployed and activated, but will have no visual impact.