Just in case, there is the code for MasterPage.master.
vb:
Partial Class MasterPage
Inherits System.Web.UI.MasterPage
Protected Sub lstPreferredTheme_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles lstPreferredTheme.SelectedIndexChanged
Dim preferredTheme As HttpCookie = New HttpCookie("PreferredTheme")
preferredTheme.Expires = DateTime.Now.AddMonths(3)
preferredTheme.Value = lstPreferredTheme.SelectedValue
Response.Cookies.Add(preferredTheme)
Response.Redirect(Request.Url.ToString())
End Sub
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not Page.IsPostBack Then
Dim selectedTheme As String = Page.Theme
Dim preferredTheme As HttpCookie = Request.Cookies.Get("PreferredTheme")
If preferredTheme IsNot Nothing Then
selectedTheme = preferredTheme.Value
End If
If lstPreferredTheme.Items.FindByValue(selectedTheme) IsNot Nothing Then
lstPreferredTheme.Items.FindByValue(selectedTheme) .Selected = True
End If
End If
Select Case Page.Theme.ToLower()
Case "darkgrey"
Menu1.Visible = False
TreeView1.Visible = True
Case Else
Menu1.Visible = True
TreeView1.Visible = False
End Select
End Sub
End Class