Do I need to add the ground wires to plastic retro fitting lights?
I have some panel lights similar to these. The fixture cases are plastic and don't need to be grounded since you couldn't anyway.

Additionally, they are powered by low voltage DC so again don't need to be grounded.

The power supplies for the ones I have only have two wires feeding them, a hot wire and a neutral. No ground, since they are also all plastic.