1 Add Skin file in to folder like below
2 Create DropDownList
<asp:dropdownlist id="ChooseTheme" runat="server" autopostback="true">
<asp:ListItem Value="Theme1">Theme1</asp:ListItem>
<asp:ListItem Value="Theme2">Theme2</asp:ListItem>
</asp:dropdownlist>
<asp:ListItem Value="Theme1">Theme1</asp:ListItem>
<asp:ListItem Value="Theme2">Theme2</asp:ListItem>
</asp:dropdownlist>
3 Coding Behind Default Page
Protected Sub Page_PreInit(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreInitPage.Theme = Request("ChooseTheme")
End Sub