simple code to change theme on run time

Here is the most simple code to change the themes dynamically.
Here I am assuming that you have three themes named Red, Green and Blue, and you have three buttons named RedBtn, GreenBtn and BlueBtn to change theme on run time. I set Green as my initial theme.

    protected void Page_PreInit(object sender, EventArgs e)
    {
      
        if((string)Session["Flag"]=="true")
        Page.Theme =(string) Session["themeSelected"];
            
    }

    
    protected void RedBtn_Click(object sender, EventArgs e)
    {   
       
        Session["Flag"] = "true";
        Session["themeSelected"] = "Red";
        Server.Transfer(Request.FilePath);
    }
    protected void GreenBtn_Click(object sender, EventArgs e)
    {
        Session["themeSelected"] = "Green";
        Server.Transfer(Request.FilePath);
    }
    protected void BlueBtn_Click(object sender, EventArgs e)
    {
        Session["Flag"] = "true";
        Session["themeSelected"] = "Blue";
        Server.Transfer(Request.FilePath);

    }