You want to make Dark Theme your default theme, but it does not persist across starting your Workspace.
Restartable Workspaces allow the setting to persist for the lifetime of the Workspace, but it's not permanent (this applies to Domino 4.4 and older versions. Note that for VSCode settings are synced and restored in 4.5 and later but could potentially be stored in a similar manner)
Find your appropriate path to the lab/settings folder:
jupyter lab paths
Create this file in 4.4.1:
With this content:
"theme": "JupyterLab Dark"
Your Workspace will now start with the Dark Theme. If you have access to the Dockerfile, you can do this with the postRun or with RUN commands.