At the bottom of the screen, tap the "Settings" icon to access the Settings menu.
In the settings menu, click on "Theme" and a window will appear where you can choose which theme you'd like to use. "System Default" sets your app to use whatever the default theme is for your app. "Light Mode will provide you with a light background, regardless of what your device defaults are set to. "Dark Mode" will provide you a dark background, regardless of what your device defaults are set to.