Just an update, I have enabled FINEST logging and found out that it was not matching because it never makes a GET to the /App_Themes folder. I suspect this is because it was already redirecting to the internal web server (since it's configured as a reverse proxy). If that's the case, how do I go about changing the paths when it's not even requesting from the external web server? Should the url rewrite be configured in the internal web server instead?
Please help me. Thank you very much!