Offer (or document) some "no theme" themes for RTF output

[Jennifer (Jenny) Bryan]

Inspired by a recent chat with @DavisVaughan re: exerting a bit of control over highlighting for pasting into a talk

We already document how to use pre-existing themes:
https://reprex.tidyverse.org/articles/rtf.html#highlight-options

But sometimes that is not enough or too much.

In the "not enough" department, I have written up how to create and use a custom theme (also linked from the article):
https://github.com/jennybc/debugging/blob/master/how-to/highlight-custom-theme.md

But it might be nice to create a theme or at least document how to create and use some "no theme" themes, i.e. for when you're happy to apply colour in Keynote or whatever the destination software is.

You could imagine wanting a theme that's just "all black" or "all white" or "black (or white) code, grey output".

I think reprex could ship with themes and then that path could presumably be provided to the highlight. Requires a bit of experimentation.