In Page_PreInit(object sender, EventArgs e)  Event ...

Unfortunately there is not built in support to change page 
themes at runtime. Here is a simple code which can be used 
to change page themes at runtime:
At first though we may say we can easily achieve this by 
coding it in Page_Preinit Event as shown below. 

protected void Page_PreInit(object sender, EventArgs e)
    {
       Page.Theme = "Black"
       
    }

But problem with this is we cant assign value from dropdown 
box because Page_Preinit event is fired much before 
dropdown has changed value.

To resolve this issue, just use the following steps:
1-Create one session variable which will hold current theme 
value 
2-On selection change event of dropdown combo box , assign 
value form combo box to session variable. 
3-During Page_preInit Event assign this variable value to 
Page.Theme property. 
4-Stop page loading and reload same page again using 
server.transfer method as shown below 


protected void Page_PreInit(object sender, EventArgs e)
    {
        string thm;
        thm = (string)Session["themeSelected"];
        if (thm != null)
        {
            Page.Theme = thm;
            DropDownList1.Text = thm;
        }
        else
        {
            Session["themeSelected"] = DropDownList1.Text;
            Page.Theme = "Blue";
        }

protected void DropDownList1_SelectedIndexChanged(object 
sender, EventArgs e)
    {
        Session["themeSelected"] = DropDownList1.Text;
        Server.Transfer(Request.FilePath);

    }