/// <summary>
/// Returns an application Url that's Theme relative.
/// images/users.gif
/// returns: /myapp/app_themes/{activetheme}/images/users.gif
/// </summary>
public static string ResolveThemeUrl(string themeRelativePath)
{
    string theme = null;
    
    // try to pick up active theme
    Page page = HttpContext.Current.Handler as Page;
    if (page != null)
        theme = page.Theme;
    
    // if not available use config default
    if (!string.IsNullOrEmpty(theme))
        theme = App.Configuration.Theme;
                
    return HttpContext.Current.Request.ApplicationPath + "/".Replace("//", "") + "app_themes/" +
           theme +  "/" +
           themeRelativePath;
}