Add darkred color for accessiblity

Author: bigtiger

src/css/rubycentral.css

@@ -3,6 +3,7 @@
 
 :root {
   --red: #E14943;
+  --darkred: #cf2821; /* For Contrast with white text */
   --purple: #755384;
   --blue: #6288BC;
   --green: #71C09E;