When Low Light Mode is enabled in OnSong the background is black and the words are white.  This feature is great for live performances where the stage lights are dimmed and the regular bright white background might be distracting.  It's one of those really useful and cool features of OnSong that you might not know about unless you go exploring deep into the settings menu.  For OnSong 2.0 and the Android version, features like this will be just under the surface and easier to access.  For now, this great tutorial by Brandon explains how to turn the Low Light Mode On and Off, plus reversing the menu scheme for better low light viewing as well.