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).