improve visibility of callout in styles.

[homberghp]

Summary

The callouts in the current dark background code snippets are not distinctive enough.
Improve the css for this element (.hljs-number).

Brief explanation of the feature.

Basic example

Old version showed it like this

My local version (no tweaks to style, but also no semantic html)

Current version:

Motivation

The color scheme is quite subtle, so some cleared demarcation between code and documentation tags (callouts)
would be helpful.

[homberghp]

Note that the color of the callout is almost identical to the color of the variable names.
Maybe make the style bold, block mode with border or outline of distinct color, borderrounding 50%