In addition to the Dask example that @jtp pointed out. There is this example by @ian-r-rose that only requires a requirements.txt file to impost restriction to JupyterLab 1.0 or greater. Some users may find that an easier solution to adapt because you can easily add additional dependencies there. Plus, the repo includes how to control the theme setting as an example of controlling other settings.
I will mention though that although my workspace.json file spelled out a rather simple set up with just four terminal panes, I had to import it in the start step like the Dask example. The build would fail with not locating the jupyterlab-workspace.json file for some reason when I put the import command in the postBuild step, like in @ian-r-rose’s example. When I moved it back to start, it worked again.