asp.net传值问题

如何从一个页面的DropDownList控件里选择一个值，传递给自身，本页面获取该值，刷新页面，呈现不同的外观
如这里有个文件，代码如下：
<%@ Page Language="VB" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><script runat="server">    Protected Sub Page_PreInit(ByVal sender As Object, ByVal e As System.EventArgs)
        
        Dim ThemeString As String
        
        ThemeString = Me.Request.QueryString("ThemeStr")
        Me.Response.Write(ThemeString)              取不到传递的值
        
        If ThemeString Is Nothing Then
            
            ThemeString = "redTheme"
            
        Else
            
            ThemeString = Me.Request.QueryString("ThemeStr")
            
            Me.Response.Write(ThemeString)
            
            'Me.Response.End()
            
        End If
        
        Me.Response.Write(ThemeString)
        
        Me.Theme = ThemeString
        
       
        Dim item As ListItem = Me.DropDownList1.Items.FindByValue(ThemeString)
        
        If item IsNot Nothing Then
            
            item.Selected = True
                   
        End If
    End Sub    Protected Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
        
        Dim url, ThemeString As String
        
        ThemeString = Me.DropDownList1.SelectedValue
        
        url = Me.Request.Path.ToString & "?ThemeStr=" & ThemeString
        
        Me.Response.Write(url)    值从这传递过去
        
        'Me.Response.Write(ThemeString)   传递的参数值
        
        Me.Response.End()
        
        Me.Response.Redirect("url")
        
    End Sub
</script><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>动态加载主题示例</title>
</head>
<body>
    <form id="form1" runat="server">
    <div style="text-align: center">
        <h1 style="text-align: center">
            <span style="color: #0000ff">
            动态加载主题示例</span></h1><hr />
        <br />
        <asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged" AutoPostBack="True">
            <asp:ListItem></asp:ListItem>
            <asp:ListItem Value="RedTheme">红色方案</asp:ListItem>
            <asp:ListItem Value="BlueTheme">蓝色方案</asp:ListItem>
            <asp:ListItem Value="GreedTheme">绿色方案</asp:ListItem>
            <asp:ListItem Value="PurpleTheme">紫色方案</asp:ListItem>
            
        </asp:DropDownList>
        &nbsp;&nbsp;<br />
        &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
        <asp:Calendar ID="Calendar1" runat="server" Height="191px" Width="485px"></asp:Calendar>
        
    </div>
    </form>
</body>
</html>
如何才能取到该值...
谢谢