为什么我的CSS在添加到项目之前不起作用？

I have an asp.net mvc project, and I replaced the default css the wizard built with the iui css and javascript package. They were placed in ~/Content/iui/ folder. When i ran the site through the dev web server, the css didnt get used. I managed to fix it by adding the iui folder to the project in the project explorer. That's all I did, the pages already the script and style tags set up properly.I have an asp.net mvc project, and I replaced t