content: Update code-block syntax highlighting styles for light mode

[chrisbobbe]

In 2023-12, web changed the set of colors it uses for syntax highlighting in code blocks in light mode. That was after we first implemented light-mode syntax highlighting here, so we're out of sync with web.

However, as Greg points out in #689 (comment) , the discussion on web's change petered out with some open feedback, and no firm conclusion on what we'd do next.

So, let's first await a color scheme in web that's reasonably stable/happy, then translate it to zulip-flutter.