Fix some theming-related issues

[ialokim]

• fixes Default theme setting is ignored #913 by changing the default to "auto" (follow system)
• fixes Background color of changelog is dark when in light mode. #909 by only checking for system-wide theme if "auto" is enabled

[Altonss]

It works fine for me (Android 13). It would be good to check also on other versions (older also if possible).
Ping @MatsG23, this should fix your issue :)

[MatsG23]

I wanted to test the changes to see if my issue is resolved but the PR does not contain a build artifact and I could not install my self-built APK (probably because it is unsigned). Anyways, I am also not using an older version of Android as well.

[ialokim]

Just tested locally on a Android 5 (API 21) powered emulator. Fixed a bug where dark mode would not be recognized for Android <10 when battery saver mode is enabled. Everything else worked as expected.