Leonidas -- you can use whatever UI framework you like, as long as it will work with a standard Cordova (aka PhoneGap app). The Intel XDK creates a standard Cordova app, so the rules that apply to Cordova apps apply to Intel XDK apps. You will have to build your UI by hand, since App Designer has been revised to only support Twitter Bootstrap (which you can also use manually). In other words, you do not have to use App Designer to build your UI, you can build it manually as most people do when building web apps for use in a browser.
