The app has light, dark, system default, AMOLED AND Material You color mode. You can change to dark theme in settings. Here you can also change the accent color.
Changing Colors Step by Step
Open the Player Main Menu
Select Settings
Scroll down to colors
Here you can choose the main background color. The options are light which has a white canvas or dark which has a dark grey canvas, and system default which adapts to your device settings.
Selecting dark changes the app to be in dark theme mode
Default dark theme
To get a true black background choose AMOLED. AMOLED is the best option if battery usage is a concern as it uses less power than the other choices.
The app with a true black background in AMOLED
From here you can also customize the accent color:
By opening the Color picker you can choose an accent color
Dark mode with green accent
Light mode with pink accent
There is also the option called Material You which will automatically adapt the app colors to your phone's wallpaper colors. Note that you need to enable this in your device's settings for it to work. Material You ads additional color schemes defined by Google and changes the colors of the app to match your wallpaper.