I have that tank too! But anyway, I'd be worried about a white substrate getting dirty. But that's because I'm pretty lazy and wouldn't feel like cleaning it from time to time. I think a black substrate could look nice with the contrast against the white it's on. I also agree that a green plant for background would look nice against the wall. Maybe a darker green, though, is what I'd say. I'm partial to dwarf hairgrass, myself. HC is always nice in a nano.
But I'm just rambling. Good luck to you.