Hi Michael，
As replied in the ticket #2511 ( in Appeon Support Center)：
Appeon supports the transparent background of the object in the Datawindow object but doesn't support the transparent property of DWO itself and the system controls. And Appeon didn't do any special handling on Mobile, so the difference between Mobile and web is depending on the OS itself.
And we'll document it in our next release.
Regards,
Appeon Support Team