App.xaml.cs

using System.Reflection;
using System.Windows;
using System;

namespace NorthHorizon.Samples.SystemThemeChange
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        public App() : this(Environment.GetCommandLineArgs()) { }

        public App(string[] args)
        {
            //SetTheme("luna", "normalcolor");
            //ThemeHelper.SetTheme("luna", "normalcolor");

            if (args.Length >= 2)
            {
                string theme = args[1], color = null;

                if (args.Length >= 3)
                    color = args[2];

                ThemeHelper.SetTheme(theme, color);
            }
        }

        /// <summary>
        /// Sets the WPF system theme.
        /// </summary>
        /// <param name="themeName">The name of the theme. (ie "aero")</param>
        /// <param name="themeColor">The name of the color. (ie "normalcolor")</param>
        public static void SetTheme(string themeName, string themeColor)
        {
            const BindingFlags staticNonPublic = BindingFlags.Static | BindingFlags.NonPublic;

            var presentationFrameworkAsm = Assembly.GetAssembly(typeof(Window));

            var themeWrapper = presentationFrameworkAsm.GetType("MS.Win32.UxThemeWrapper");

            var isActiveField = themeWrapper.GetField("_isActive", staticNonPublic);
            var themeColorField = themeWrapper.GetField("_themeColor", staticNonPublic);
            var themeNameField = themeWrapper.GetField("_themeName", staticNonPublic);

            // Set this to true so WPF doesn't default to classic.
            isActiveField.SetValue(null, true);

            themeColorField.SetValue(null, themeColor);
            themeNameField.SetValue(null, themeName);
        }
    }
}