Themes are styling objects in asp.net. We can set the themes by using Theme page directive or by adding or modifing <pages theme=”" /> tag Web.config.
But if you want to change it in run-time you need to set the theme in the Page_PreInit event in the ASP.Net Page Life-Cycle.

Simple implementation of changing theme in dynamically in asp.net

In this there is a dropdown list which displays all the theme / skin files in the current application, with autopostback enabled.

Page_Load event

protected void Page_Load(object sender, EventArgs e)
  {
  if (!Page.IsPostBack)
  {
  foreach (string dir in Directory.GetDirectories(Request.MapPath("./App_Themes")))
  {
  this.ThemeSelector.Items.Add(new ListItem(Path.GetFileNameWithoutExtension(dir), Path.GetFileNameWithoutExtension(dir)));
  }
  this.ThemeSelector.Items.Insert(0, new ListItem("-- SELECT --", "-1"));
  }
  }

Dropdown selected Index change Event
protected void ThemeSelector_SelectedIndexChanged(object sender, EventArgs e)
  {
  if (this.ThemeSelector.SelectedValue != "-1")
  {
  Session["theme"] = this.ThemeSelector.SelectedValue;
  }
  }
Page_PreInit event
protected void Page_PreInit(object sender, EventArgs e)
  {
  if (Session["theme"] != null)
  {
  this.Theme = Session["theme"] as string;
  }
  }

The selected theme is storing into a session variable in selected Index change event and in the Page preinit event it is setting as the current page’s theme. This can be applied to all the pages in a web application, because we are using Session variable.