2024.6 or higher
Theme options: Light, Dark, Automatic
Android Tap A (Chord Chart settings), either choose App Theme (Light, Dark, Automatic) or Song Theme for options
iOS  Tap Aa (Chord Chart settings) > Theme
2023.7 or below
In song view, tap on Chord chart settings (third button from the right on the top in song view), then Colors.
You can change the color of the chord symbols and the background.