Mobile apps unit screens flash from light to dark background

[vunguyen-dmt]

The issue
This makes users feel extremely uncomfortable using dark mode, thus making the dark mode almost unusable.
I opened the issue here rather than in the app repositories because I think the solution is from the backend.

Reproduce
Set your mobile device theme to dark theme, open openedx mobile app, open a course, and go to unit screens (it's easier to see in units containing text components), you will see the screen flash from white to dark theme background.
You can use edx.org apps to test.

Suggestion:
The dark theme background or class should be set on the server side, the theme mode can be extracted from the request cookie or a variable can be used in the same user' preferred language is handled.
The color code for the background can be set through a config variable with a default value.