message: Fix highlighting for users visiting in private mode
diff: 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;
  