Set colour in highlighted items in table of content

Currently the items in table of contents are highlighted as bold based on items being
viewed on the screen or as the user scrolls up/down on a page. Added the `primaryColor`
of the site's configuration to make the current section being viewed standout more
in the table of content.

closes #1608

packages/docusaurus-1.x/lib/static/css/main.css

@@ -1979,6 +1979,7 @@ input::placeholder {
 
 .onPageNav .toc-headings > li > a.active {
   font-weight: 600;
+  color: $primaryColor;
 }
 
 .onPageNav ul {