When playing in a dark environment, switch to a Dark option to reduce the brightness of the screen. 
 
2023.11 or higher: Tap the Chord chart settings icon, iOS [Aa] Android [A] (top, towards the right)
Select from the options in Theme
 
 
2023.7 or below: When playing in a dark environment, switch to Night mode to reduce the brightness of the screen. Tap the Chord chart settings icon (top, towards the right)
iOS [Aa]; Android [cog wheel icon, left of the pencil icon, as shown below]