var _1200RDUri = new Uri(String.Format(@"/aa;Component/Themes/1200Theme.xaml"), UriKind.RelativeOrAbsolute);

            var _980RDUri = new Uri(String.Format(@"/aa;Component/Themes/980Theme.xaml"), UriKind.RelativeOrAbsolute);

            var _1200RD = Application.LoadComponent(_1200RDUri) as ResourceDictionary;

            var _980RD = Application.LoadComponent(_980RDUri) as ResourceDictionary;

            if (_1200RD == null || _980RD == null)

            {

                return;

            }

            if (!IsScreenWidthAbove1200)

            {

                Application.Current.Resources.MergedDictionaries.Remove(_1200RD);

                Application.Current.Resources.MergedDictionaries.Remove(_980RD);

                Application.Current.Resources.MergedDictionaries.Add(_980RD);

            }

            else

            {

                Application.Current.Resources.MergedDictionaries.Remove(_1200RD);

                Application.Current.Resources.MergedDictionaries.Remove(_980RD);

                Application.Current.Resources.MergedDictionaries.Add(_1200RD);

            }

 private static void ChangeAppStyle(ResourceDictionary resources, Tuple<AppTheme, Accent> oldThemeInfo, Accent newAccent, AppTheme newTheme)

        {

            var themeChanged = false;

            if (oldThemeInfo != null)

            {

                var oldAccent = oldThemeInfo.Item2;

                if (oldAccent != null && oldAccent.Name != newAccent.Name)

                {

                    var key = oldAccent.Resources.Source.ToString().ToLower();

                    var oldAccentResource = resources.MergedDictionaries.Where(x => x.Source != null).FirstOrDefault(d => d.Source.ToString().ToLower() == key);

                    if (oldAccentResource != null)

                    {

                        resources.MergedDictionaries.Add(newAccent.Resources);

                        resources.MergedDictionaries.Remove(oldAccentResource);

                        themeChanged = true;

                    }

                }

                var oldTheme = oldThemeInfo.Item1;

                if (oldTheme != null && oldTheme != newTheme)

                {

                    var key = oldTheme.Resources.Source.ToString().ToLower();

                    var oldThemeResource = resources.MergedDictionaries.Where(x => x.Source != null).FirstOrDefault(d => d.Source.ToString().ToLower() == key);

                    if (oldThemeResource != null)

                    {

                        resources.MergedDictionaries.Add(newTheme.Resources);

                        resources.MergedDictionaries.Remove(oldThemeResource);

                        themeChanged = true;

                    }

                }

            }

            else

            {

                ChangeAppStyle(resources, newAccent, newTheme);

                themeChanged = true;

            }

            if (themeChanged)

            {

                OnThemeChanged(newAccent, newTheme);

            }

        }

这段不能直接用，得看一下代码意思

Application.Current.Resources
 Resources = new ResourceDictionary {Source = resourceAddress};