Use slightly lighter gray in headings.

services/web/public/stylesheets/core/variables.less

@@ -80,7 +80,7 @@
 @headings-font-family:    @font-family-serif;
 @headings-font-weight:    500;
 @headings-line-height:    1.1;
-@headings-color:          @gray-darker;
+@headings-color:          @gray-dark;
 
 
 //-- Iconography