ASP.NET 4.0 CSS in a folder is not cached

ASP.NET 4.0 CSS in a folder is not cached

I converted an existing website from ASP.NET 2.0 to ASP.NET 4.0.
Now when I see the network tab in Firebug I see the css files in my
root/css folder are retrieved upon each request with 200 OK response. So
they are not cached.
Although the css files I have in my aspnet Theme are getting cached. So I
moved the /css folder files in Themes folder and those files are now
cached.
Why are styles in folder other than Themes aren't cached? I wonder if I am
missing some configuration setting?
P.S.: I am running the new website from VS2010/DevServer/FireFox.