Overlays are currently bugged with video threading in RetroNX .
But a workaround is in place!
Put a 1280x720px PNG file into /retroarch/overlays/<core name as in the menu bottom>.png
That includes spaces!
Currently u cant swap/disable them In-Game.
Pre-Made Overlays made by the libretro community:
https://github.com/libretro/overlay-borders
Make sure to scale them properly!
Not that tricky to get them working.