Provide a default implementation of setPreferredTheme

And only override it if needed

app/src/main/java/me/impy/aegis/ui/ScannerActivity.java

@@ -87,11 +87,6 @@ public class ScannerActivity extends AegisActivity implements ZXingScannerView.R
         }
     }
 
-    @Override
-    protected void setPreferredTheme(boolean darkMode) {
-
-    }
-
     @Override
     public void onResume() {
         super.onResume();