Reset CSS cache by forcing CSS files to refresh when using ASP.NET Themes

resetcss.cs

protected override void OnPreRender(System.EventArgs e)
{
  //loop through all the controls
  foreach (Control oLink in Page.Header.Controls){
      if (oLink is HtmlLink){        
        HtmlLink cssLink = oLink as HtmlLink;        
        //Check if CSS link 
        if (cssLink.Attributes["type"].Equals("text/css", StringComparison.CurrentCultureIgnoreCase)){          
          if (cssLink.Attributes["href"].Contains(String.Format("~/App_Themes/{0}/", Page.Theme))){
            //add a version of your app here.                                                                
            cssLink.Attributes["href"] += "?v.1";
          }
        }
      }
  }
  base.OnPreRender(e);
}

For active development (change CSS, refresh browser, rinse, repeat), changing that version number to a RNG should clear it every page load.