Removed DayNight theme. Added a 'white' theme for Light, and 'black' theme for Dark.
Users will have to set the type of Light or Dark theme they want to use.
Default is white for light, black for dark.
More themes will be added later.
This commit sets up the base for future enhancements in direct messages. The changes are:
1. Introducing fragments and navigation using androidx.navigation.fragment.NavHostFragment.
2. Using ListAdapter and LiveData for the RecyclerViews.
3. Fixes a bug where a single direct message was lost when querying older messages.
