I've created a new project that only includes a very simple activity that uses the new Day/Night theme.
Even when calling AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_NO);
in a static block on the application class AND calling getDelegate().setLocalNightMode(AppCompatDelegate.MODE_NIGHT_NO);
in the activity, the activity is showing the night theme.
I've set a break point on the activity and its reporting that the UI mode is night, so I've got no idea why it's refusing to show the day theme.
I'm compiling against/targetting API 23 and testing on a Nexus 5 running Marshmallow