how to add css class to html generic control div?

I created a div tag like this:

System.Web.UI.HtmlControls.HtmlGenericControl dynDiv = 
    new System.Web.UI.HtmlControls.HtmlGenericControl("DIV");

I added style to the div tag like this:

dynDiv.Style.Add(HtmlTextWriterStyle.BorderStyle, "1px solid #DBE0E4");
dynDiv.Style.Add(HtmlTextWriterStyle.Height, "auto");
dynDiv.Style.Add(HtmlTextWriterStyle.MarginTop, "5px");
dynDiv.Style.Add(HtmlTextWriterStyle.PaddingBottom, "5px");
dynDiv.Style.Add(HtmlTextWriterStyle.Width, "682px");

But I need to control the style of the div tag through an external css file located at folder ~/css/maincss.css.

How can I apply the css in that file to this div?