Follow system dark mode by default (#1674)

Author: SUPERCILEX

app/src/main/java/com/firebase/uidemo/FirebaseUIDemo.java

@@ -2,9 +2,14 @@
 
 import com.squareup.leakcanary.LeakCanary;
 
+import androidx.appcompat.app.AppCompatDelegate;
 import androidx.multidex.MultiDexApplication;
 
 public class FirebaseUIDemo extends MultiDexApplication {
+    static {
+        AppCompatDelegate.setDefaultNightMode(AppCompatDelegate.MODE_NIGHT_AUTO_BATTERY);
+    }
+
     @Override
     public void onCreate() {
         super.onCreate();

app/src/main/java/com/firebase/uidemo/auth/AuthUiActivity.java

@@ -347,7 +347,7 @@ private void startSignedInActivity(@Nullable IdpResponse response) {
     @OnClick({R.id.default_theme, R.id.purple_theme, R.id.green_theme, R.id.dark_theme})
     public void toggleDarkTheme() {
         int mode = mDarkTheme.isChecked() ?
-                AppCompatDelegate.MODE_NIGHT_YES : AppCompatDelegate.MODE_NIGHT_NO;
+                AppCompatDelegate.MODE_NIGHT_YES : AppCompatDelegate.MODE_NIGHT_AUTO_BATTERY;
         AppCompatDelegate.setDefaultNightMode(mode);
         getDelegate().setLocalNightMode(mode);
     }