Fix contrast issues with comments in code examples

The colours were updated to better match version 6
but for comments they were mistakenly set to the wrong colour.

This results in comments that are very hard to read.

I have put in a quick patch here to resolve the issue immediately and will
upstream this patch into the tech-docs-gem.

alphagov/tech-docs-gem@f09a4b8#diff-81435f36c443c406a3436b0531a4991cd20655917ede750cec10d5b2c6b3bdcf

Author: NickColley

source/stylesheets/screen.css.scss

@@ -4,3 +4,12 @@
 .technical-documentation .table-container td > code {
   white-space: nowrap;
 }
+
+// Temporarily override syntax highlighting to fix poor contrast for comments.
+.highlight .c,
+.highlight .cm,
+.highlight .cp,
+.highlight .c1,
+.highlight .cs { /* Comment.Special */
+  color: govuk-functional-colour(secondary-text);
+}