Android: Option to change colour theme from system default

[micmalti]

First of all, thank you for open-sourcing this project. Having the ability to self-host down the road was one of the key reasons why I switched over to Omnivore from Pocket.

In the Android version, the colour theme automatically follows the system default, with no option of changing it except from within the reading view, though any change is limited to this view only.

Understandably, there are other issues which require more attention than expanding the app theming options, so I think it would be enough for the time being to grant users the option to choose between the two currently available themes. A potential way of doing this is by replacing isSystemInDarkTheme() with a DarkThemePref() mapped to an 'Enable dark mode' checkbox in the profile settings.