Improve code syntax contrast (#79)

## Summary

- switch Hugo/Chroma highlighting from the light GitHub theme to
github-dark
- keep highlighted code blocks consistent with the existing Prism
Tomorrow token palette
- update the theme comment to reflect that highlighted blocks no longer
always use a light background

## Testing

- npm run build
- verified the affected PSRule article renders code with a dark
background and high-contrast text

Closes #74

Author: eosfor

hugo.yaml

@@ -19,7 +19,7 @@ markup:
     renderer:
       unsafe: false
   highlight:
-    style: github
+    style: github-dark
     lineNos: true
     codeFences: true
 
@@ -186,4 +186,4 @@ related:
   - name: tags
     weight: 80
   - name: categories
-    weight: 60
+    weight: 60

themes/powershell-community/layouts/_default/baseof.html

@@ -175,7 +175,7 @@
            terminal chrome renders flush; Prism handles token colors. */
         .ps-terminal pre { background: transparent; color: #ccc; margin: 0; border-radius: 0; padding: 1.25rem; overflow-x: auto; }
 
-        /* Hugo/Chroma highlighted blocks (.highlight) carry their own light
+        /* Hugo/Chroma highlighted blocks (.highlight) carry their own
            background; trim the heavier .prose pre padding/margins so they don't
            read as a wall of whitespace. */
         .prose .highlight { margin: 1.25rem 0; border-radius: 0.5rem; overflow: hidden; }
@@ -361,4 +361,4 @@
     {{ block "scripts" . }}{{ end }}
 </body>
 
-</html>
+</html>