Add opaque background to search results section

app/javascript/styles/components.scss

@@ -2990,6 +2990,10 @@ button.icon-button.active i.fa-retweet {
   font-weight: 500;
 }
 
+.search-results__section {
+  background: lighten($ui-base-color, 2%);
+}
+
 .search-results__hashtag {
   display: block;
   padding: 10px;