Improve inline code rendering in Notebooks (light mode)

Created by: benvenker

Syntax highlighting would be great, but even just making the background higher contrast would help. Right now it's very hard to distinguish inline code from regular text:

It looks like we have some CSS precedent in our docs formatting of larger code blocks in light mode: https://docs.sourcegraph.com/admin/how-to/enable-experimental-feature

@novoselrok we had chatted about maybe fixing this and https://github.com/sourcegraph/sourcegraph/issues/2144 at the same time. Just posting this here so we don't forget to pull both in.