Styles.Render in MVC4

It's calling the files included in that particular bundle which is declared inside the BundleConfig class in the App_Start folder.

In that particular case The call to @Styles.Render("~/Content/css") is calling "~/Content/site.css".

bundles.Add(new StyleBundle("~/Content/css").Include("~/Content/site.css"));

A bit late to the party. But it seems like no one has mentioned
bundling & minification of StyleBundle, so..

@Styles.Render("~/Content/css") 

calls in Application_Start():

BundleConfig.RegisterBundles(BundleTable.Bundles);            

which in turn calls

public static void RegisterBundles(BundleCollection bundles)
{
    bundles.Add(new StyleBundle("~/Content/css").Include(
              "~/Content/bootstrap.css",
              "~/Content/Site.css"));
}

---

RegisterBundles() effectively combines & minifies bootstrap.css & Site.css
into a single file,

<link href="/Content/css?v=omEnf6XKhDfHpwdllcEwzSIFQajQQLOQweh_aX9VVWY1" rel="stylesheet">

But..

<system.web>
  <compilation debug="false" targetFramework="4.6.1" />
</system.web>

only when debug is set to false in Web.config.
Otherwise bootstrap.css & Site.css will be served individually.
Not bundled, nor minified:

<link href="/Content/bootstrap.css" rel="stylesheet">
<link href="/Content/Site.css" rel="stylesheet">

Watch out for case sensitivity. If you have a file

/Content/bootstrap.css

and you redirect in your Bundle.config to

.Include("~/Content/Bootstrap.css")

it will not load the css.