If you’re hosting your game on Dashingdon, you can do this easily enough by going into the game’s settings (on Dashingdon) and customizing what appearance you want to choose. (This includes a dark background with white font.)
However, that only applies to the game as long as it’s being hosted and played on Dashingdon, so to permanently change the background of your game, you’ll need to edit the game’s Style.css file. You can find out how to do that here. There is also a color change mod that someone made here. Hope that helps!