A GXY map has UI to change the background color under the Map → Colors → Background…​. menu item.

You can make sure that all your maps have a custom background color by adjusting the background color of the map in the TLcyMapComponentFactory. 

public class CustomMapComponentFactory extends TLcyMapComponentFactory {

  @Override
  protected ILcdGXYView createGXYView(TLcyMapComponent aMapComponent) {
    //Ask the super method to create the view
    ILcdGXYView view = super.createGXYView(aMapComponent);
    //Set a custom background color on the view
    view.setBackground(Color.RED);
    return view;
  }
}

Alternatively, you can use a listener to adjust the background color after the map has been created:

ILcyGenericMapManagerListener<ILcdView, ILcdLayer> listener =
    ILcyGenericMapManagerListener.onMapAdded(ILcdGXYView.class, mapComponent -> mapComponent.getMainView().setBackground(Color.RED));
aLucyEnv.getCombinedMapManager().addMapManagerListener(listener);