Fix highlighting for users visiting in private mode

Author: jyn514

templates/macros.html

@@ -30,6 +30,7 @@
             case "ayu":
                 stylesheet = "https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.4.0/styles/dark.min.css";
                 break;
+            case "null": // The user is visiting docs.rs for the first time and hasn't set a theme yet.
             case "light":
                 stylesheet = "https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.4.0/styles/github.min.css";
                 break;