Use a distinct color for selected autocomplete items

This restores the ability to see which autocomplete item is selected (which
seems to have gotten lost during redesign).

Fixes https://github.com/vector-im/riot-web/issues/9134

res/css/views/rooms/_Autocomplete.scss

@@ -71,7 +71,7 @@
 
 .mx_Autocomplete_Completion.selected,
 .mx_Autocomplete_Completion:hover {
-    background: $menu-bg-color;
+    background: $selected-color;
     outline: none;
 }