Refresh syntax-highlighting style from upstream, add separate dark styles

FeRD (Frank Dana)requested to merge
ferdnyc/ui-bundle:hljs-styles into stg

Replace the outdated highlight.css file with the current upstream github.css and github-dark.css styles (direct from the highlight.js source repo), renamed to hljs-github{,-dark}.css.

Customize the hljs-github-dark.css file for the Fedora Docs site (by adding .dark in front of all of the selectors).

Apply (different) green text shades for .hljs-link and .hljs-tag (unstyled in upstream CSS).

Merge request reports