See https://ithoughthecamewithyou.com/post/minify-and-inline-css-for-aspnet-mvc

InlineCss.cs

public ActionResult InlineCss()
{
    BundleContext context = new BundleContext(
        new HttpContextWrapper(System.Web.HttpContext.Current), 
        BundleTable.Bundles, 
        "~/Content/css");
            
    Bundle cssBundle = BundleTable.Bundles.GetBundleFor("~/Content/css");
    BundleResponse response = cssBundle.GenerateBundleResponse(context);
           
    CssMinify cssMinify = new CssMinify();
    cssMinify.Process(context, response);

    return Content(response.Content);
}