HTMLManager: Do not reset lab's CSS variables if they are defined

[martinRenou]

Do not override the JupyterLab's CSS variables if they are already defined in the page.
This fixes the nbconvert's HTMLExporter when using a dark theme.

Testing it:

jupyter nbconvert --to html --HTMLExporter.theme=dark --HTMLExporter.template_name=lab --HTMLExporter.widget_renderer_url=/path/to/local/ipywidgets/packages/html-manager/dist/embed-amd.js NotebookWithWidgets.ipynb

Screenshot:

References:

Fix #3340 fix jupyter/nbconvert#1506

[github-actions]

👈 Launch a binder notebook on branch martinRenou/ipywidgets/fix_html_manager_css

[martinRenou]

As a side note, I wonder if this logic should not be in the base-manager so that we get it for free in any manager (thinking about Voila for example, which does not use the html-manager and needs to manually inject lab variables in the classic theme).

[maartenbreddels]

See also #2534

[martinRenou]

Thanks for the pointer! I didn't see your PR

[maartenbreddels]

Yeah, I also had trouble finding it, but I remember trying to attack a similar issue.

[jasongrout]

Thanks!

[martinRenou]

meeseeksdev backport to 7.x