Unfortunately, there is no (simple) way to do this.
The reason is that the object is ThemeDisplayattached to the current user and the current page. In the context of the scheduler (i.e., an automatically triggered event created by the system itself), there is no current registered user (normal, you can use the default user, but ...), and there is no page displayed.
. , . , PortalUtil.getPortalURL() .
, ThemeDisplay.