code: protected void fadeInCurrentTheme() {
          if (currentTheme != null) {
              currentTheme.setLooping(false);
              currentTheme.setVolume(0f);
              currentTheme.setOnCompletionListener(listener);
              stage.addAction(
                      VolumeAction.setVolume(currentTheme, 0f, musicVolume.getPercent(), duration, Interpolation.fade));
              currentTheme.play();
          }
      }
docs: Smoothly fades in current theme.