Failing to place an object in front of a scene

Hello community,
I am facing a difficulty to place the orientation gizmo over the scene I have created. the library from this gizmo is found here:

And the repository of my code is this:

I appreciate any help!

To be more specific, I would like to overlap two objects in mount. I have appended it but I cannot see one over the other. They end up one on the top of the other or under.

I just have forgotten to import the css into the app.js