The best way to implement a theme is to make a theme extension and publish it to npm. Such as this one GitHub - oriolmirosa/jupyterlab_materialdarker: The Material Darker theme for JupyterLab. In that way, all js and css code will be hidden and you can switch between themes easily via Settings → Jupyterlab Theme.
I’m not sure that there is any other way to hide js code.
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