I can produce the above result in Mapsource and Basecamp by adding to the map's TYP file the polygon 0x4b (Map Background or Data Bounds) without any Draw Order.
When I transfer this map to my 3490, 205w, GarminMobilePc and Streetpilot Android the result is that this polygon is shown transparent (????)
When I do the same to a nonNT map, the same polygon is shown as solid, filled with the color of my choice.
It's like the NT map cannot read the 0x4b polygon, like there is another polygon for Map Background when the map is NT.

But if I use 0x10d01 polygon as Map Background, things start to look promising!!!
I will look into it....
Spoiler: 3490 screenshots

The first screenshot is the map with default TYP and Garmin Theme (transparent background, 3D basemap is shown or the yellowish color on other devices that do not support 3D)
The second screenshot is the map with changed TYP and German Theme
The third screenshot is the map with Changed TYP and Garmin Theme
The custom color for the background is green-yellow

ncb2