To enable Dark Music Mode:
Open any song part in your ensemble.
Tap the screen to reveal the Beam Button at the bottom right.
Tap the Beam Button and toggle on 'Dark Music Mode.'
This will invert the colors of the pdf, resulting in a black background and the notes and type being white. Annotations that were made on the music will not invert in color.
Dark Music Mode will stay on for all parts that you open until you toggle it back off. You are able to be in dark music mode while viewing parts on your own and while in Live Mode.