Do you know where
dialogue.css comes from?
I do not believe that this file comes with Management Server so it must be a part of your project. 403 Forbidden usually means that the file or folder it is in has been configured to be blocked on the web server, or requires a user with correct permissions.
A few things to look into to try to get to the bottom of the issue:
Does it work in Page Preview?
Does it work in Smart Edit?
Does it work on the published site?
Does it affect all users?
When did the issue start?
What do you see in the IIS log? It should show where the server expected to find the file, letting you map that to the real drive on the server to investigate.