code: private void initializeStage() {
          // Define the stage title
          this.stage.setTitle(applicationTitle());
  
          // Define stage icons, the toolkit will use the best size
          final List<Image> stageIcons = stageIcons();
          if (stageIcons != null && !stageIcons.isEmpty()) {
              this.stage.getIcons().addAll(stageIcons);
          }
  
          // and allow customization
          customizeStage(this.stage);
      }
docs: Customize the primary Stage.