CSS doesn't know anything about the File.DirAssets constant or any of B4J's unique syntax. This works for me:
TButt.Style = TButt.Style & "-fx-background-image:url('/Files/1434671913_Robot_Devil.png');"
when the Robot Devil image is in the Files folder in the B4J project. You should also make sure the image is synced in the Files Tab in the IDE so that it is properly packaged into the jar file.
When I do this, it properly displays transparency in the Robot Devil png. That is, I can set the background color of the button using CSS and it will show through on the transparent parts of the png. The downside to this CSS approach, though, is that you have to manually size the png to your button.