Set search result lists font colour to black

https://www.drupal.org/project/admin_toolbar/issues/3522548

Increased search result items color contrast by changing the default color to be darker.

Updated search field placeholder label to include shortcut defined introduced in MR !141 (merged).

admin_toolbar_search/css/admin.toolbar_search.css

@@ -72,6 +72,7 @@
 }
 
 .admin-toolbar-search-autocomplete-list .ui-menu-item a {
+  color: #333333;
   text-decoration: none;
 }