Improve HighlightJS CSS for JSON

[toedter]

In dark mode, only white for keys and blue for values,
in light mode everything is black.

It would be great if JSON would be a bit more "colorful", e.g. also render numbers in another color than strings.

[philwebb]

Thanks for the suggestion, I've tweaked the CSS a little. It now looks like this:

It's still not that colorful, but it's closer to the GitHub look:

[toedter]

Thx @philwebb