WPF Theme Manager

ThemeManager.cs

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

public static class ThemeManager
{
    #region Fields

    private const BindingFlags DefaultStaticFlags = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Static;
    private const BindingFlags DefaultInstanceFlags = BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;

    private static readonly Assembly PresentationFramework = typeof(FrameworkElement).Assembly;

    private static readonly Type UxThemeWrapper = PresentationFramework.GetType("MS.Win32.UxThemeWrapper");
    private static readonly Type SystemResources = PresentationFramework.GetType("System.Windows.SystemResources");

    private static readonly MethodInfo SystemResources_InvalidateResources = SystemResources.GetMethod("InvalidateResources", DefaultStaticFlags);
    private static readonly MethodInfo SystemResources_OnThemeChanged = SystemResources.GetMethod("OnThemeChanged", DefaultStaticFlags);
    private static readonly MethodInfo SystemParameters_InvalidateCache = typeof(SystemParameters).GetMethod("InvalidateCache", DefaultStaticFlags, null, Type.EmptyTypes, null);
    private static readonly MethodInfo SystemColors_InvalidateCache = typeof(SystemColors).GetMethod("InvalidateCache", DefaultStaticFlags);

    private static readonly FieldInfo UxThemeWrapper_isActive = UxThemeWrapper.GetField("_isActive", DefaultStaticFlags);
    private static readonly FieldInfo UxThemeWrapper_themeColor = UxThemeWrapper.GetField("_themeColor", DefaultStaticFlags);
    private static readonly FieldInfo UxThemeWrapper_themeName = UxThemeWrapper.GetField("_themeName", DefaultStaticFlags);

    #endregion

    #region Intercept Theme Change

    static ThemeManager()
    {
        SystemResources.GetMethod("EnsureResourceChangeListener", DefaultStaticFlags).Invoke(null, null);

        var hook = Delegate.CreateDelegate(typeof(DependencyObject).Assembly.GetType("MS.Win32.HwndWrapperHook"),
            typeof(ThemeManager).GetMethod(nameof(FilterThemeMessage), DefaultStaticFlags));

        var notify = SystemResources.GetField("_hwndNotify", DefaultStaticFlags).GetValue(null);
        notify = notify.GetType().GetProperty("Value", DefaultInstanceFlags).GetValue(notify, null);
        notify.GetType().GetMethod("AddHook", DefaultInstanceFlags).Invoke(notify, new object[] { hook });
    }

    private static IntPtr FilterThemeMessage(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        if (msg == 0x31a) // WM_THEMECHANGED
        {
            handled = true;
        }
        return IntPtr.Zero;
    }

    #endregion

    #region Change Theme

    /// <summary>
    /// Changes the theme using a compound theme name (theme-name[.theme-color]).
    /// </summary>
    /// <param name="compoundThemeName">Compound theme name.</param>
    public static void ChangeTheme(string compoundThemeName)
    {
        if (string.IsNullOrEmpty(compoundThemeName))
        {
            ChangeTheme(string.Empty, string.Empty);
        }
        else
        {
            var themeData = compoundThemeName.Split('.');
            ChangeTheme(themeData[0], themeData.Length == 1 ? string.Empty : themeData[1]);
        }
    }

    /// <summary>
    /// Changes the theme.
    /// </summary>
    /// <param name="themeName">Name of the theme.</param>
    /// <param name="themeColor">Color of the theme.</param>
    public static void ChangeTheme(string themeName, string themeColor)
    {
        SystemColors_InvalidateCache.Invoke(null, null);
        SystemParameters_InvalidateCache.Invoke(null, null);

        SystemResources_OnThemeChanged.Invoke(null, null);

        if (!string.IsNullOrEmpty(themeName))
        {
            UxThemeWrapper_isActive.SetValue(null, true);
            UxThemeWrapper_themeName.SetValue(null, themeName);
            UxThemeWrapper_themeColor.SetValue(null, themeColor);
        }

        SystemResources_InvalidateResources.Invoke(null, new object[] { false });
    }

    #endregion
}

This overrides the default OS theme. Although it is quite a hack (uses private Reflection) it has several advantages. See this post.