How can I make a black background with white text in OnSong?
This is what is known as Low Light Mode.
You can enable Low Light Mode to make white text appear on a black background. However, please keep in mind that this will only work with text-based files and not PDF, JPG, or other non-text-based files.
To do this, simply open the Style Preferences menu in the menubar (the sliders icon next to the headphones icon) > tap the Document tab > tap the circle icon under the Mode section. This will toggle on and off Low Light Mode. You can also toggle on and off Low Light mode by going to the Utilities gear > Settings > Display Settings > Low Light Mode.