Step 1 -
Have a look at below link to fetch your Data from SQL Server Database.
http://msdn.microsoft.com/en-us/library/bh8kx08z.aspx
Step 2 -
Fetch your details from Database in
Page_Init
Event and register your CSS reference as below.
protected void Page_Init(object sender, EventArgs e)
{
string cssHrefFromDB = "cssHrefFromDB";
HtmlLink cssDynamic = new HtmlLink();
cssDynamic.Href = cssHrefFromDB;
cssDynamic.Attributes["rel"] = "stylesheet";
cssDynamic.Attributes["type"] = "text/css";
cssDynamic.Attributes["media"] = "all";
Page.Header.Controls.Add(cssDynamic);
}