Hi, ari. I am afraid the only way should being select check box "use App Deisgner" while you start a new project. And the system would pop-up a window ask you to select a proper Framework of UI design. If you do not select to use App Designer, the system would not help you to insert css, font and js files for relevant framework.
1. You may cannot access App Designer if you created a project without using this GUI layout tool.
2. For the Blank Template, enable the App Designer layout GUI editor by checking Use App Designer. Other Templates do not support selecting App Designer (Layout and User Interface group and the Games group).
3. And you cannot enable App Designer when importing an HTML5 code base.
Learn more information about App Designer, please visit following link:
Hope it could help you.
There is no simple way to convert a non-App Designer app into an App Designer app, it does not have an import feature. You have to start as an App Designer project. If you have non-UI code that you want to import, you can simply copy it into the new App Designer project.