UPDATE 02/03/2018
Now this option is already built in in the settings menu!
Recently this issue was closed on the GitHub, and finally it is possible to edit settings of the jupyter lab. Hurray!
The procedure is described in this pull request.
Just update your jupyter-lab (now it is version 0.27.0) and go to File - Settings, then select Theme tab. And type your theme name in the User Overrides and press the check-mark at the right of the string. I tried several, but only "theme": "JupyterLab Dark" seems to work. Enough for me.
I really appreciate the work the jupyter team is doing. Thank you all!
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