PR: Implement dark theme

[jitseniesen]

This PR renders notebooks using the JupyerLab dark theme if the Spyder interface is set to dark.

It was not such a big change in the end but I learned a lot about JupyterLab, TypeScript and webpack in the process. You will see that the colour scheme of the JupyterLab dark theme is a bit different from Spyder: the text of the notebook pane is whiter and the background is blacker than in the other panes.

[codecov-commenter]

Codecov Report

Merging #297 into master will increase coverage by 0.48%.
The diff coverage is 100.00%.

Impacted Files	Coverage Δ
spyder_notebook/notebookplugin.py	77.54% <100.00%> (+0.24%)	⬆️
spyder_notebook/server/main.py	100.00% <100.00%> (ø)
spyder_notebook/utils/nbopen.py	90.19% <100.00%> (+0.40%)	⬆️
spyder_notebook/widgets/notebooktabwidget.py	85.10% <100.00%> (+0.43%)	⬆️

[ccordoba12]

@jitseniesen, it looks pretty awesome! Keep up with the good work here.

[kingwatam]

For some reason, spyder-notebook remains to have a white background even when Spyder is set to dark. I installed the latest spyder-notebook from conda.

[jitseniesen]

@kingwatam Can you please check the version of spyder-notebook that is reported in Help > Dependencies? The latest version is 0.3.1. One possible explanation is that Spyder is not using the new version; perhaps it got installed in the wrong environment.

[kingwatam]

I couldn't find spyder-notebook under dependencies, even though I had the notebook tab in Spyder. However, I could find spyder-notebook in Anaconda Navigator, version 0.2.3. That must be why.

After uninstalling Spyder (4.0.1), and reinstalling it (4.1.5), as well as reinstalling spyder-notebook (0.3.1), it now works as expected. Thanks for the help!