You can get the room center and emit anything you like from the theme.
To do that, Add the attached blueprint to your dungeon actor’s Marker Emitter list. Then create a marker in your theme file named “RoomCenter” and your objects will show up in the center of the room. If you want to preview this in the theme editor, add this in the theme editor’s dungeon properties too
For corners you can use Spatial constraints.
https://.youtube.com/watch?v=eL4C42A7rx4
You can add variations to various rooms in your levels using dynamically placed theme override volumes (2.2.0+)