Applying Dynamic Themes to WebPage

Applying Dynamic Themes to Web Page (.aspx) in ASP.NET

Themes and Skins. The themes and skins features in ASP.NET 2.0 allow for easy customization of your site's look-and-feel. You can define style information in a common location called a "theme", and apply that style information globally to pages or controls in your site. Like Master Pages, this improves the maintainability of your site and avoid unnecessary duplication of code for shared styles.

protected void Page_PreInit(object sender, EventArgs e)
{
  Page.Theme = Session["Theme"] as string ?? "Theme1";
}

protected void Page_Load(object sender, EventArgs e)
{

}
protected void Button1_Click(object sender, EventArgs e)
{
  if (Session["Theme"] != null)
  {
      if (Session["Theme"].ToString() == "Theme1")
          Session["Theme"] = "Theme2";
      else
          Session["Theme"] = "Theme1";
  }
  else
  {
      Session["Theme"] = "Theme2";
  }
  Server.Transfer(Request.FilePath);
}

for more information related Theme & CSS class Click here