Providing a custom.css file to mimic Jupyter's styles

[gbrault]

Is your feature request related to a problem? Please describe.
All Jupyter style are gone using html_nocode

Describe the solution you'd like
Could you distribute a custom.css file that would mimic Jupyter's one?

[timkpaine]

@gbrault this shouldn't be necessary since JupyterLab styles are distributed in NBConvert itself (and some parts in jupyterlab pygments). Can you see if you're still missing styles on v0.3.1?