It worked!
Now, I wanted to modify the opacity because I intended to use a picture as background, so I put
@target_window.opacity = 255
@itemback= Sprite.new(@viewport)
@itemback.bitmap = Cache.system("Gameover")
@itemback.x = 0
@itemback.y = 0
@itemback.z = 0
In line 1198 or so (most certainly this isn't the correct method).
EDIT: The problem now is that I can't see the background picture below the target window.
Oh, I forgot to say, that for some reason
YEM Item Overhaul now only works in conjunction with
Centered Command Window.
It's in the new Melody demo, along with some unfinished scripts. It's still named "1.00m", but it's different from the old 1.00m version.