feat: use primary color for highlighted items in table of contents (f…

…acebook#1628)

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 facebook#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 {