Thanks Bryan - that seems like it’s exactly what I’d want, but I could not get it to work.
Based on this post, it sounds like GlobalInlineStyleSheet may not work…
Best workaround I’ve been able to come up with is to define a collection of widgets and then loop through those with a function that sets the stylesheets and css_classes properties of each widget. Bit clunky, but does the trick…
(I also again tried to define something in jupyter’s custom_css, but that also didn’t work - probably because of the :host thingy 
)
Thanks again for all your help!