Use a fainted text color for <hr> elements in the landing page (#4443)

* Use a fainted text color for <hr> elements in the landing page

* remove trailing whitespace

app/javascript/styles/about.scss

@@ -309,6 +309,10 @@
     margin-bottom: 0;
   }
 
+  hr {
+    border-color: rgba($ui-base-lighter-color, .6);
+  }
+
   .header {
     line-height: 30px;
     overflow: hidden;