I\'ve found out several topics related to this issue yet I couldn\'t find an answer which worked for my problem, which is following: I moved my Asp.Net 4.0 site to a new server
The articles/topics that you have found is talking about the same thing as Uali's answer, which is correct, but it doesn't always solve everyone's problem. The problem is also not about your css per se.
Before proceeding, make sure that you have enabled Static Content as the other answer instructed.
Then do these steps in the IIS Manager window with your virtual directory selected.
Update: I also forgot to say that you can do the same thing for .jpg, .png, .gif or any other file types if necessary.