You can absolutely make a backlit sign with a dark background. I assume you are referring to 'halo' illuminated RPC letters? Obviously the brightness of the glow around the letters is quite a bit less than a similar sign on a lighter color background, thats just the result of the dark color absorbing the light. You could make the letters metallic gold and add a thin painted aluminum piece the same size or slightly larger than the letter mounted to the face of the background so the light has a lighter surface to reflect off of. see pic as example