How can I get rid of the solid background in an animation

Two steps are needed:

1.: set alpha: true when creating the renderer:

renderer = new THREE.WebGLRenderer( { antialias: true, alpha: true, } );

2.: define a background image in your .css:

`canvas {
  display: block;
  background: url(./Test.jpg) repeat 0 0;
  background-size: auto;
}
`

Depending on how you want to fill the background yourself, you may get away without step 2.

See an example of mine where I show three.js graphics against a copyright screen. More details on the project itself here (also on this forum).