docs: Improve dark mode syntax highlighting (#18002)

This PR introduces [GitHub
Light](https://github.com/highlightjs/highlight.js/blob/main/src/styles/github.css)
and [GitHub
Dark](https://github.com/highlightjs/highlight.js/blob/main/src/styles/github-dark.css)
as the syntax highlighting themes for the corresponding modes.

Release Notes:

- N/A

docs/theme/css/general.css

@@ -355,7 +355,7 @@ kbd {
   font-style: italic;
 }
 
-code.hljs {
+code:not(pre code).hljs {
   color: var(--code-text) !important;
-  background-color: var(--code-bg);
+  background-color: var(--code-bg) !important;
 }