There can be issues with colour quality or legibility. For example, black text on a white screen becomes white text on a black screen. Colour inversion applies to everything on your device, including media.Colours don't change in media, such as videos. Dark theme applies to the Android system UI and supported apps.
You can change your display to a dark background by using Dark theme or colour inversion. Some of these steps only work on Android 11.0 and up.