Hi,
check this once I hope it can useful for you
<% if (Session["userRole"].ToString() == "Guest") {
%>
<link href="Styles/basic.css" rel="stylesheet" type="text/css" />
<%
}
else if (Session["userRole"].ToString() == "RootAdmin") {
%>
<link href="Styles/basic_ie.css" rel="stylesheet" type="text/css" />
<%
} else if (Session["userRole"].ToString() == "Admin") {
%>
<link href="Styles/demo.css" rel="stylesheet" type="text/css" />
<%
}
else if (Session["userRole"].ToString() == "User") {
%>
<link href="Styles/Site.css" rel="stylesheet" type="text/css" />
<%
} %>
All the Best