There’s a difference here, and a big one, diannepat. There’s “change the background colour of text”, and there’s “pigment text based on keywords within a code fence”.
We’ve discussed code fences here a lot, and while I think they’re considering it down the line, as they say, it’s not a markdown editor. I’d love to see a true code fence instead of preformatted, and allow something like python Pygments to do the work, but I’m finding that sort of thing exceeds what Agenda sets out to do. You might find joy in Typora (all md) as a close option, or Quiver (json+), but I think the most you’ll see in the near term is just colour-behind-text, probably in system colour.