Use dark mode-themed code blocks on dark mode [Edge Docs]

[MatheusRich]

I believe it's more natural to keep the code blocks theme matching. It can cause unnecessary contrast changes when we put a light-themed code block on a page with mostly dark backgrounds.

Here's what the guides currently look:

Here's what the guides would look like with a Monokai theme:

Prior work

Both Python and Elixir docs do similar things (i.e., dark mode code blocks on dark mode theme).

[AmandaPerino]

@MatheusRich And this one - please close this issue since it is addressed in #51372. Thank you so much for your input!