The answer to this question points out that the "JupyterLab Dark" theme is now available as part of a vanilla jupyter-lab install.
How can I configure jupyter-lab so that it launches with this theme already applied on the first launch, so I don't have to manually select it every time I launch my jupyter docker container?
As the answer from mike and the comment from MarkusOdenthal did not work for me when I used the jupyter docker stacks I looked for a different solution and it seams using overrides.json is the current recommended approach:
https://jupyterlab.readthedocs.io/en/stable/user/directories.html#overridesjson
In short: check the application directory with jupyter lab path and place a file named overrides.json containing
{ "@jupyterlab/apputils-extension:themes": { "theme": "JupyterLab Dark" } } into <application directory>/settings/ (e.g. /opt/conda/share/jupyter/lab/settings/ for the official jupyter docker containers)
So if you want to have the base image from jupyter with dark theme, the Dockerfile would be
FROM jupyter/base-notebook COPY overrides.json /opt/conda/share/jupyter/lab/settings/ Put this in ~/.jupyterlab/user-settings/@jupyterlab/apputils-extension/themes.jupyterlab-settings
{ "theme": "JupyerLab Dark" } In the Dockerfile this might look like:
RUN mkdir -p ~/.jupyterlab/user-settings/@jupyterlab/apputils-extension/ && \ echo '{ "theme":"JupyterLab Dark" }' > themes.jupyterlab-settings 
~/jupyter/lab/user-settings... now with the extra slash. And whilst the mkdir makes the directory, the echo actually writes in the current directory if you don't cd there first.Based on @mike's answer + @Mark Setchell's comment,
ARG USER=docker_user RUN mkdir -p /home/$USER/.jupyter/lab/user-settings/@jupyterlab/apputils-extension/ && \ echo '{ "theme":"JupyterLab Dark" }' > /home/$USER/.jupyter/lab/user-settings/@jupyterlab/apputils-extension/themes.jupyterlab-settings For me the '~' to replace /home/$USER did not work as well
Important note: The .jupyterlab folder seemed to have changed to .jupyter/lab, as commented in the original answer
