Just stumbled on this myself, so I don't know if you allready found the answer and the question is stil relevant.
I also did not manage to get the above Rendertheme working, and indeed, a blanc screen with no Errors. I think thats probably because Mapsforge handels Errors internally.
And did not know where to put the symbols and patterns folder as this is required in the above theme : <file:/path/to/symbol/icon/hotel.png" />
What works however is to copy and edit the rendertheme that is allready present within the Mapsforge library.
I have succesfully added Cycling nodes for the Netherlands to it.
You can find the original Rendertheme 'osmarender.xml' inside the jar when you extract its contents (OSMDroid_4_1_MapsForgeTileSource\assets\rendertheme\osmarender.xml)
You can then load your new rendertheme with the code you allready supplied
Dim InputStream1 As InputStream
InputStream1 = File.OpenInput(File.DirAssets, "my-osmarender.xml")
MapsForgeTileSource1.GetMapsForgeTileSourceOptions.SetRenderTheme(InputStream1)
Kind regards,
Patrick