go to your App_Start Folder > BundleConfig.Cs > then find the code block that will look similar to this
bundles.Add(new StyleBundle("~/Content/themes/base/css").Include(
"~/Content/themes/base/jquery.ui.core.css",
"~/Content/themes/base/jquery.ui.resizable.css",
"~/Content/themes/base/jquery.ui.selectable.css",
"~/Content/themes/base/jquery.ui.accordion.css",
"~/Content/themes/base/jquery.ui.autocomplete.css",
"~/Content/themes/base/jquery.ui.button.css",
"~/Content/themes/base/jquery.ui.dialog.css",
"~/Content/themes/base/jquery.ui.slider.css",
"~/Content/themes/base/jquery.ui.tabs.css",
"~/Content/themes/base/jquery.ui.datepicker.css",
"~/Content/themes/base/jquery.ui.progressbar.css",
"~/Content/themes/base/jquery.ui.theme.css"));
then add the path of your css file in here
edit 1
Now you have advised that you are using an empty preview template you have two options,
Add the App_start folder and add the above code block and then in your Layout.cshtml file add the line
@Styles.Render("~/Content/themes/base/css")
or
- in your Layout.cshtml file reference is as you would normally, but youll need it in your shared view so it carries through the rest of the site
<link href="~/css/css.css" rel="stylesheet" />is working fine. But I have it in the _Layout.cshtml file, I think it should work.