Custom Action for CSS

customactioncss.cs

public override void FeatureActivated(SPFeatureReceiverProperties properties)
{  
   //Add the Css to the current Web
    SPSite site = properties.Feature.Parent as SPSite;
    SPWeb web = site.OpenWeb();
    if (web.ServerRelativeUrl == "/")
    {
        web.AlternateCssUrl = web.ServerRelativeUrl + "SiteAssets/Styles/mystyles.css";
    }
    else
    {
        web.AlternateCssUrl = web.ServerRelativeUrl + "/SiteAssets/Styles/mystyles.css";
    }
    web.Update();
}