Select JupyterLab Theme automatically when the docker starts

[bollwyvl]

In your docker build, you can bake in an ${NB_PREFIX}/share/jupyter/lab/settings/overrides.json (docs):

{
 "@jupyterlab/apputils-extension:themes": {
    "theme": "JupyterLab Dark",
    "theme-scrollbars": true
  }
}