I don't think so. I searched "chat" in the gui folder and nothing related to the chat background showed up. The high contrast resource pack doesn't change it. If you want to change the font, it's possible. I think the background is controlled by the chat settings.
I don't know how to make a biome but maybe I could try. If this comment wins, I could maybe put the structures in different biomes (for example a mansion on a mountain)