Theming: enable day/night mode incl. app-wide setting.

src/main/java/de/dennisguse/opentracks/Startup.java

@@ -6,6 +6,7 @@ import android.util.Log;
 
 import java.util.Locale;
 
+import de.dennisguse.opentracks.util.ActivityUtils;
 import de.dennisguse.opentracks.util.PreferencesUtils;
 
 /**
@@ -32,6 +33,8 @@ public class Startup extends Application {
             PreferencesUtils.setString(this, R.string.stats_units_key, statsUnits);
         }
 
+        ActivityUtils.applyNightMode(this);
+
         //TODO Workaround to reset recordingTrackId on app startup as the TrackRecordingService (likely) crashed.
         if (PreferencesUtils.isRecording(this)) {
             Log.e(TAG, "Reset recordingTrackId; likely the TrackRecordingService crashed.");