Python: Improve theme color application in Help code snippets

[petetronic]

With vscode styles injected as vars, we could improve the styling of code snippets in pydoc help by mapping some syntax token colors through to the markup used in pydoc for code.

[petetronic]

Follow-on work from #430

[seeM]

We'll first need to inject those token colors as CSS variables before we can use them in our CSS file.