Fix dark theme's code block background, line number colors (#1124)

When customizing `$code-background-color` in `dark.scss`, the result is a multi-color background. (see https://github.com/just-the-docs/just-the-docs/issues/1121#issuecomment-1374976843)

This makes OneDarkJekyll code block colors the same as the specified `$code-background-color`, and uses the `$default-body-color` as the line number text color, which can otherwise be invisible due to the default being black and is hard to see on a very dark code block background.

_sass/code.scss

@@ -207,4 +207,21 @@ code.language-mermaid {
   border: 0;
 }
 
+// Override OneDarkJekyll Colors for Code Blocks
+.highlight,
+pre.highlight {
+  background: $code-background-color; // Code Background
+  // For Backwards Compatibility Before $code-linenumber-color was added
+  @if variable-exists(code-linenumber-color) {
+    color: $code-linenumber-color; // Code Line Numbers
+  } @else {
+    color: $body-text-color; // Code Line Numbers
+  }
+}
+
+// Override OneDarkJekyll Colors for Code Blocks
+.highlight pre {
+  background: $code-background-color; // Code Background
+}
+
 // {% endraw %}