i would check the file you are modifying (site.css) is infacted the file being linked into the _Layout.cshtml (or whatever other root template youre using)
"<link href="@Url.Content("~/Content/SiteTypoHere.css")" rel="stylesheet" type="text/css" />"
also load the page in FireFox use firebug and select the heading it will give u a line number ref to the CSS code and filename it is ACTUALLY using to render
hope one of these helps you.