Implement Dark Theme (DayNight with automatic option)

This is just a first rough implementation.
A real UI designer should look over this.

briar-android/src/main/res/layout/fragment_blog.xml

@@ -6,6 +6,5 @@
 	xmlns:tools="http://schemas.android.com/tools"
 	android:layout_width="match_parent"
 	android:layout_height="match_parent"
-	android:background="@color/window_background"
 	app:scrollToEnd="false"
 	tools:context=".android.blog.BlogActivity"/>