Lua Code:
Health:SetFrameLevel(10)
Health.backdrop:SetFrameLevel(8)
Power:SetFrameLevel(Health:GetFrameLevel() - 4)
Power.backdrop:SetFrameLevel(Health:GetFrameLevel() - 6)
SetFrameLevel only supports numbers between 0-20 so you also need to make sure to not give it a smaller/bigger numbers.
I would also suggest to always use at leas 2 level diferrences between frames due a strata bug in the game, and also you might want to show something between that two frame layers.