Add missing background for CORE-292

Author: jmgasper

src/shared/components/challenge-detail/Header/style.scss

@@ -18,7 +18,7 @@
   font-size: 12px;
   font-family: 'Roboto', sans-serif;
   line-height: 1.2em;
-  background-color: $tc-gray-20;
+  background-color: #efefef99;
   padding-left: 20px;
   padding-right: 30px;
   padding-bottom: 10px;