diff --git a/notebook/app.py b/notebook/app.py
index 9a969b50a..55795b359 100644
--- a/notebook/app.py
+++ b/notebook/app.py
@@ -354,7 +354,8 @@ class JupyterNotebookApp(NotebookConfigShimMixin, LabServerApp):  # type:ignore[
         self.handlers.append(("/edit(.*)", FileHandler))
         self.handlers.append(("/consoles/(.*)", ConsoleHandler))
         self.handlers.append(("/terminals/(.*)", TerminalHandler))
-        self.handlers.append(("/custom/custom.css", CustomCssHandler))
+        if self.custom_css:
+            self.handlers.append(("/custom/custom.css", CustomCssHandler))
         super().initialize_handlers()
 
     def initialize(self, argv: list[str] | None = None) -> None:  # noqa: ARG002