This asp.net example article will Demonstrates you  how to Dynamically set a link to a CSS file in ASP.NET .For Dynamically set a link to a CSS file in ASP.NET follow the below steps.
1-For Dynamically set a link
add a runat attribute to your traditional link tag this
tag must have its ID attaribute present.
<head>
<link id="MyCSSSheet" rel="stylesheet"
type="text/css" runat="server" />
</head>
2-Now in your content page
Page_Load event , add  the  "href" attribute of the link tag as
below.
   protected void Page_Load(object
sender, EventArgs e)
    {
        if
(!Page.IsPostBack)
        {
           MyCSSSheet.Attributes.Add("href","path
of Css file");
        }
    }
