动态切换主题

            var paletteHelper = new PaletteHelper();
            //Retrieve the app's existing theme
            ITheme theme = paletteHelper.GetTheme();
            //Change the base theme to Dark
            theme.SetBaseTheme(Theme.Dark);
            //or theme.SetBaseTheme(Theme.Light);
            //Change all of the primary colors to Red
            theme.SetPrimaryColor(Colors.Red);
            //Change all of the secondary colors to Blue
            theme.SetSecondaryColor(Colors.Blue);
            //You can also change a single color on the theme, and optionally set the corresponding foreground color
            theme.PrimaryMid = new ColorPair(Colors.Brown, Colors.White);
            //Change the app's current theme
            paletteHelper.SetTheme(theme);