VB.NET

        Dim Theme As String
        Dim ThemeConfig As System.Configuration.Configuration
        Dim section As PagesSection

        ThemeConfig = System.Web.Configuration.WebConfigurationManager.OpenWebConfiguration("/")

        section = ThemeConfig.GetSection("system.web/pages")
        section.Theme = Theme

        ThemeConfig.Save()

        Response.Redirect("~/Default.aspx")