Jupyter conversion is using out of date CSS

[sean-fitzpatrick]

The CSS for the Jupyter conversion is from the mathbook era.

This becomes noticeable in particular if one is viewing a notebook using the dark mode in Jupyter lab: the CSS sets a white background, and the dark mode makes the text white as well.

I've confirmed that if I copy the css links from a current PreTeXt HTML build and paste them into the first cell of the Jupyter notebook, it fixes the white background (and presumably a whole host of other things).